Mistakes in the Harry Potter books

"As obsessive fans will tell you, I do slip up! Several classrooms move floors mysteriously between books and these are the least serious continuity errors! Most of the fansites will point you in the direction of my mistakes. But the essentials remain consistent from book to book because the story has been plotted for a long time and it is clear in my mind."

- J. K. Rowling on the errors within the franchise

Below is a list of mistakes that occur in the Harry Potter book series. Some of these mistakes were fixed in later editions of the same book.

Cover

 * The art for the cover of the original UK edition depicts Harry at Platform Nine and Three-Quarters wearing a Gryffindor scarf, however, Harry hasn't been sorted yet. It is possible, however, that the illustration is of Harry returning to Little Whinging at the end of the school term.
 * The cover art of the original UK edition depicts another train beside the Hogwarts Express.
 * The 15th Anniversary US cover by Kazu Kibuishi depicts Hagrid and Harry with only four fingers on their right hand.

Chapter 1: The Boy Who Lived

 * The beginning of the story takes place on a "dull, grey Tuesday". But in 1981, 31 October was on a Saturday.
 * Wizards are said to be celebrating the fall of Voldemort all day long, yet the Potters were attacked at night. Unless it took Hagrid a full day to get Harry from his parents' house in Godric's Hollow to Little Whinging, this seemingly does not make sense. Although, it does not say when Hagrid retrieved Harry from the house – it could have been a full day until then, or he could have kept Harry with him until night when he could fly under the cover of darkness. However, the day is 1 November, so the celebrations are happening after his downfall, which was the evening prior.
 * Dumbledore says that he passes through several parties to get to Little Whinging, but he got there by Apparition, which is a direct form of transport. Although, he could have apparated several times to go to many parties – possibly in order to lose anyone (such as Death Eaters) who might have been following him.
 * In the Bloomsbury edition, on page 17, Hagrid says "I'll be takin' Sirius his bike back". But in Harry Potter and the Prisoner of Azkaban, it states that Sirius told Hagrid to keep the bike because he didn't need it anymore. In later editions of this book, Hagrid says "I'd best get this bike away" instead.

Chapter 2: The Vanishing Glass

 * The trip to the zoo is said to take place on a Saturday, but Dudley's eleventh birthday (June 23, 1991) was actually on a Sunday.
 * The boa constrictor at the Zoo supposedly winked at Harry during their conversation. As snakes don't have eyelids, it is impossible for them to wink.

Chapter 3: The Letters from No One

 * Harry's 11th birthday (31 July, 1991) was said to be on a Tuesday. But in 1991 in the real world, that date was a Wednesday.

Chapter 4: The Keeper of the Keys

 * Petunia speaks of Lily "turning teacups into rats" during her holidays away from Hogwarts. Yet since underage wizards aren't permitted to use magic outside of school, this claim is highly unlikely and would be more likely to fit Petunia's anti-magic mentality.

Chapter 5: Diagon Alley

 * On Harry's Hogwarts list of school supplies, "1 Wand" is listed twice, as both the first and last item on the list. This mistake was fixed in later editions of the book. The list of school supplies also doesn't include potion ingredients, although Harry later buys them.
 * Harry has to buy Fantastic Beasts And Where To Find Them, but he doesn't start his Care of Magical Creatures classes until his third year. It is possible however, that the book may have been used for other classes.
 * Harry buys the book One Thousand Magical Herbs and Fungi, but later, he looks up dittany in "One Hundred Magical Herbs and Fungi". This error was, however, amended in later editions.
 * The air in the tunnels to the Gringotts vaults is described as getting colder as they go deeper. This only happens very near the surface. Once you get below the level where the temperature depends heavily on the weather at the surface, the geothermal gradient takes over and it gets hotter as you go deeper.
 * When Harry is talking to Hagrid about his first meeting with Draco Malfoy, Hagrid says that "there's not a single witch or wizard who went bad that wasn't in Slytherin." At the point that this conversation takes place, Hagrid would have been under the impression that Sirius, a Gryffindor, was responsible for betraying Harry's parents. It is probable, however that Hagrid was merely exaggerating.
 * When Harry returns from Diagon Alley to Little Whinging, he takes a train from Paddington Station. But Little Whinging is in Surrey, south of London, so he should have travelled from Victoria or Waterloo; trains from Paddington head to the west.

Chapter 6: The Journey from Platform Nine and Three-Quarters

 * Aunt Petunia is described as looking as surprised as the other Dursleys about Harry needing to board at Platform Nine and Three-Quarters, yet according to Harry Potter and the Deathly Hallows, she had been there at least once. Although, she may have been pretending to be startled in order to distance herself from the magical world.


 * On his search for Platform Nine and Three-Quarters, Harry overhears Molly Weasley asking her children "Now, what's the platform number?" to which Ginny replies "Nine and three-quarters!". Considering that she must have been on that platform many times, it seems odd that she can't remember its number. It is probable however, that she was just trying to make Ginny feel included.
 * On page 79, the closing quotation mark is missing right after Hermione tells Harry about the books he is in. This mistake was fixed in later editions of the book.

Chapter 7: The Sorting Hat

 * While being sorted into houses, Harry looks up at the Sorting Hat's stool, and there are only three people left to be sorted. Professor McGonagall then calls out the names of four more students. This error only occurred in the US edition and was rectified in later editions – in the UK edition Dean Thomas's name is omitted.
 * During the Sorting Ceremony, Sally-Anne Perks is sorted before Harry but in Harry Potter and the Order of the Phoenix, when everyone in fifth year are being called in alphabetical order to take their practical O.W.L.s, Parvati Patil is called before Harry. It should actually have been Sally-Anne Perks, but because many students were pulled out of the school that year, it is possible that Sally-Anne was cut out.
 * When Harry first meets Nicholas de Mimsy-Porpington, the ghost says how he has not eaten in nearly 400 years. Yet he had, at the time, already been dead for 99 years longer than that. This was an error which was fixed in later editions of the book.

Chapter 9: The Midnight Duel

 * Fred tells Harry that the Gryffindor Quidditch team hadn't won a championship "since Charlie left". Charlie attended Hogwarts from 1984-1991, and so had only just graduated as Harry arrived – Harry filled the seeker vacancy left by Charlie. However, Gryffindor had not won the championship since 1986, so Fred's statement, while technically accurate, is not an accurate portrayal of the situation.

Chapter 10: Hallowe'en

 * During the Hallowe'en feast, when the troll is let in, Professor Dumbledore sends the students to their dormitories. But the troll is said to be in the dungeons, and that is also where the Slytherin dormitories are, meaning that Dumbledore has put them directly in harm's way by sending them there. The logical move would be for every student to stay in the Great Hall, do a roll call, and send teachers to go and look for missing students.

Chapter 14: Norbert the Norwegian Ridgeback

 * The night Charlie's letter shows up at Hogwarts is said to be Wednesday at midnight. However, earlier in the book it is mentioned that Astronomy classes take place every Wednesday at midnight. Which means that they are presumably skipping class, and the other students should not be asleep. This error was corrected to Saturday in later editions of the book.

Chapter 15: The Forbidden Forest

 * After Harry, Hermione, and Neville lose Gryffindor 150 House points, it is mentioned that the Gryffindor Quidditch team are refusing to speak to Harry and only refer to him as "the Seeker" if they have to refer to him at all. However, given that Fred and George are generally very laid-back, on good terms with Harry and, according to Ron, have lost numerous points for misdemeanours themselves, this does not seem fitting. It also states that Oliver Wood spoke his name, leaving the three chasers as the only ones calling him "the Seeker". It is possible that, as the reader gleans this information through Harry's state of mind, that Harry was exaggerating how hostile the team were to him, and that it was only some of the team that treated him unpleasantly.

Chapter 16: Through the Trapdoor

 * During the match against the giant chess set, Ron says "Well, Harry, you take the place of that bishop, and Hermione, you go next to him instead of that castle" But the castle is next to the knight, not the bishop. It is possible though that when Ron said "next to him" he didn't mean immediately next to, only in the general area of where Harry was located.
 * At the beginning of the chess match, Ron is the knight in the giant chess set. It later says that he had to "move ahead one" so that the Queen could take him, opening the path for Harry to checkmate the king, and win the match. If he were a knight, he would only be able to move either ahead one, and sideways two, or ahead two, and sideways one. It is possible, however, that "one" was meant as one move, rather than one space.
 * When Harry and Hermione drink the two potions to get them through Snape's task, there is only one bottle to get them through the black fire and only one bottle to get them through the purple fire. The smallest bottle holds the potion for the black fire, and it only contains enough for one person. If this is so, then it is unknown how Quirrell got to the Mirror of Erised in the first place.

Chapter 17: The Man with Two Faces

 * Dumbledore says that his and Hermione's owls crossed in mid-air, however a few pages later Hermione says to Harry that that she ran into Dumbledore on the way to the Owlery to send him the owl.
 * Dumbledore claims that the Mirror of Erised was enchanted to give the Philosopher's Stone to "one who wanted to find the stone - find it, but not use it" However, as he stated at the time, Quirrell did not want to use the stone, he wanted to present it to his master. Thus, unless Dumbledore was describing his spell too narrowly, Quirrell should have been able to retrieve the stone. It could be, however, that given that Quirrell shares his body with Voldemort, the enchantment on the mirror recognises them both as the same person. Another possible explanation is that Dumbledore enchanted the mirror so that only Harry could ever retrieve the stone, because Dumbledore did give hints to Harry about the stone throughout the story.
 * Harry reassures Ron and Hermione that his summer won't be too bad since the Dursleys aren't aware that he can't use magic outside of Hogwarts. However, Petunia grew up having a witch as a sister and these rules were established long before Lily's schooling, so she would've known about the ban on underage magic.

Cover

 * The UK version of the cover illustration of Harry, Ron, and Hedwig in the flying Ford Anglia incorrectly depicts Hedwig as a Barn Owl rather than a Snowy Owl.

Chapter 2: Dobby's Warning

 * Petunia should have been aware that underage wizards and witches are not allowed to use magic outside of school, since the rule was written in 1875, well before Lily's time at Hogwarts.

Chapter 3: The Burrow

 * After Harry, Ron, Fred, and George arrive at the Burrow, Mrs Weasley shouts at Fred first. Then she says, "and you two" glaring at Ron and Fred, but it should say George since she has already yelled at Fred. This error was corrected in later editions of the book.

Chapter 4: At Flourish and Blotts

 * The Hogwarts' equipment list mentions the Lockhart book "Wanderings with Werewolves", but later, Lockhart refers to the book as "Weekend with a Werewolf". This error was fixed in later editions of the same book.

Chapter 5: The Whomping Willow

 * When Harry and Ron arrive at Hogwarts in the flying Ford Anglia, a scene which takes place on September 1, 1992, they are told that they had been seen by Muggles in various locations, including the "Post Office Tower". But due to a change of ownership, the tower has actually been called the BT Tower since 1984. Also, the building is about a mile southwest of King's Cross, so to pass it, Harry and Ron would have had to be going in an entirely different direction to be following the Hogwarts Express.

Chapter 9: The Writing on the Wall

 * Professor Binns tells his class that the Chamber of Secrets is merely a myth. But if it had been opened before in his lifetime, he would surely believe in it.
 * Hermione says that Moaning Myrtle haunts the bathroom on the first floor. However, when Harry sees the writing on the wall outside her bathroom, he is on the second floor. This is possibly explained by the fact that in the UK, the ground floor is the floor on which the entrances are, and the floor above it is the first floor.
 * It remains unknown how Moaning Myrtle never notices Ginny visiting her bathroom, or the pipe door opening, or the Basilisk coming through her bathroom.

Chapter 11: The Duelling Club

 * When Harry speaks Parseltongue to the snake during his duel with Draco, everyone hears him hissing. Therefore, when Harry hears the Basilisk speaking in the walls, Ron and Hermione should hear hissing, yet they claim to hear nothing. A possible explanation for this could be that the Basilisk is hissing very quietly, so Ron and Hermione can barely hear it, but Harry can make out the words as he hears them in English.

Chapter 13: The Very Secret Diary

 * 14 February 1993 was a Sunday in the real world, yet students don't have classes on weekends (Harry got his musical message when he was walking upstairs for Charms Class on this day).

Chapter 14: Cornelius Fudge

 * When Harry is at the Quidditch pitch, reflecting with relief that there will be no one in the castle to become victims of the monster of Slytherin, Professor McGonagall shows up to say that the monster has struck again, and that everyone should return to their dormitories, presumably to keep them safe. However, the students would have been far safer to remain at the pitch while the professors checked the castle again.

Chapter 15: Aragog

 * When Harry and Ron go to see Aragog, they feed Fang treacle toffee to glue his teeth together. Later, "Fang suddenly let loose a great bark." It is possible, however, that the treacle toffee dissolved after a certain period of time.

Chapter 16: The Chamber of Secrets

 * It is stated that the Basilisk was able to move through the castle's pipes, but it remains unknown how something as large as the Basilisk would be able to fit through the plumbing, or where it was entering into the castle undetected in order to attack students.

Chapter 18: Dobby's Reward

 * On page 283 of some copies of the US paperback edition, the word "professor" is misspelled as "professorr".
 * Dumbledore tells Harry that Lord Voldemort is the last remaining ancestor of Salazar Slytherin. This is meant say that Lord Voldemort is the last remaining descendant of Salazar Slytherin. This error was fixed in later editions of the book.

Chapter 1: Owl Post

 * In earlier editions of the book, the first page stated that A History of Magic was written by Adalbert Waffling, rather than Bathilda Bagshot; this was rectified in later edition of the same book.

Chapter 4: The Leaky Cauldron

 * When Harry is buying his books at Flourish and Blotts, it mentions that he is being served by the manager, but when Harry is staring at the book Death Omens, it says that he is being served by the assistant of the store. Although, it is possible that the manager was simply the assistant to the shop owner.
 * Mrs Weasley complains that Fred and George have not been made prefects. However, as only one male Gryffindor prefect would have been chosen from their year, it would have been impossible for both of them to be prefects.
 * Mr Weasley says that Ron and Harry have ended up in the Forbidden Forest twice, but only Harry has ended up in the forest twice. Ron's first time into the forest was when he went with Harry to see Aragog in their second year, but in their first year, only Harry went into the forest for his detention; Ron was not there (he is in the film but not in the book).

Chapter 5: The Dementor

 * When Harry, Ron, and Hermione first find a carriage, and have stowed their luggage, they return to the platform to say goodbye to Mr and Mrs Weasley. Upon reentering the train, it is said that the three of them set off looking for an empty compartment and end up finding one at the very end of the train. Since the trio had already stowed their luggage earlier, there would be no need for them to go looking for an empty compartment. However, after Harry explains his situation with Sirius, it's revealed that Harry's Sneakoscope is in his trunk on the rack above them.
 * 1 September in 1993 was the night of a full moon. Therefore, Lupin should not have been on the train, considering how later in the book he is unable to teach for the three days around the full moon.

Chapter 6: Talons and Tea Leaves

 * Hermione's Time-Turner is a major secret that supposedly took a lot of persuasion to obtain. However, the reason it was necessary should not have been an out-of-place occurrence: any Gryffindor taking the Study of Ancient Runes would need to use one to attend their Friday Charms class that happens at the same time. Likewise, any Gryffindor who takes any combination of Divination, Muggle Studies, and Ancient Runes would require a Time-Turner since all of those classes occur at the same time on Wednesday mornings. However, one solution that Hermione's case was an unusual one. Perhaps most students would be told that they could not take those classes together in one year, but she persisted in asking for them.


 * Hermione was the only Gryffindor said to be taking extra classes, so hence, she would have to be attending classes with other Houses instead of her own. But, Ron tells Harry and Hermione in Harry Potter and the Philosopher's Stone that Percy got 12 O.W.L.s, however this is not possible unless he used a Time-Turner, as Hermione gets only 10 O.W.L.s excluding Divination and Muggle Studies.

Chapter 9: Grim Defeat

 * During his substitution of the Defence Against the Dark Arts class, Snape says "That is the second time you have spoken out of turn, Miss Granger", but actually it is the third time she speaks out of turn. Although, Snape may have not counted the first time.

Chapter 11: The Firebolt

 * Given that Remus Lupin had a transformation after Halloween but before the following weekend, the full moon would have had to be sometime in the first week of November. Since a full moon occurs every month or so, the December and January full moons would also have occurred sometime around the first week of those months. Therefore, a full moon during the Christmas period is not possible. Yet, Dumbledore explains to Trelawney that Lupin's absence from the Christmas dinner is that he is not well again; implying either a transformation or post-transformation exhaustion, especially since Snape is mentioned as having brewed his potion for him.

Chapter 16: Professor Trelawney's Prediction

 * Before Harry and Hermione go back in time, it mentions that Buckbeak is tied to a tree. But when they go back in time, Buckbeak is tied to a fence. This error was fixed in later editions of the same book.
 * This chapter takes place in June of 1994. At the beginning of that chapter, Harry Potter reads a note from Hagrid that says "Buckbeak's appeal — it's set for the sixth." The night after the hippogriff's appeal, Remus Lupin is forced into werewolf form by the full moon, which means that this happened on the night of 6 June, 1994. However, in 1994, 9 June was the night of a new moon, which means that a full moon could not possibly rise three days earlier.

Chapter 19: The Servant of Lord Voldemort

 * Snape says in the Shrieking Shack that Lupin had not taken his potion that night, but it is explained that one must drink the Wolfsbane Potion the week before transforming — not on the night itself.

Chapter 22: Owl Post Again

 * In June, Fred and George Weasley find out they have scraped "a few O.W.L.s apiece." However, in subsequent books, it is firmly established that O.W.L. results are sent home by owl and are not received until August. However, it is possible that they sneaked into an office and retrieved the information of their results, or were merely assuming that they got low marks.
 * On the American version of the cover, the front art is the night that Harry and Hermione save Sirius and Buckbeak. However on the back side of the cover (Lupin appears as a werewolf), the moon is a crescent when it should be a full moon.
 * The cover of the Scholastic edition depicts Harry and Hermione riding Buckbeak. Harry's legs are shown to be in front of Buckbeak's wings. However, when Harry was in Care of Magical Creatures and rode Buckbeak for the first time, Hagrid had told him to sit behind the wing joint (as he did in the film).
 * However, it could be because Hermione is sitting behind him.
 * Sirius Black says that Voldemort has been in hiding for fifteen years. He has actually been in hiding for twelve years (almost thirteen at the time that he says it) — about the same amount of time since Black was imprisoned. Voldemort went into hiding right after he was ripped from his body after failing to kill Harry.
 * In the last chapter on page 311, it says Dumblefore instead of Dumbledore (rectified in later editions).
 * On the back cover it says Harry blew up his horrible Aunt Petunia instead of Aunt Marge.
 * On page 379-380 of the American paperback edition, when exiting the tunnel that lead out of the Whomping Willow, it was Crookshanks, Lupin, Pettigrew, Ron, Snape, Sirius, Harry, and Hermione. Later on when Hermione used the Time-Turner (page 408), and she and Harry were watching the Whomping Willow to see themselves coming out the order changed to Lupin, Ron, Pettigrew, Hermione, the unconscious Snape, Harry, and Sirius.
 * They could have simply missed Crookshanks exiting, though there is no explanation for the change in order of the visible humans, or they were simply stating the people coming out.
 * When Harry gets back into the Gryffindor common room shortly before the Gryffindor-Ravenclaw match with his Firebolt given back to him after being tested for jinxes, the common room admires the broom. An unidentified Gryffindor tells Harry, "Ravenclaw'll have no chance, they're all on Cleansweep Sevens!" However, later in the book, Oliver Wood tells Harry that Cho Chang, the Ravenclaw Seeker, rides a Comet 260.
 * It is possible, however, that the unidentified Gryffindor who told Harry this probably doesn't know much about the Ravenclaw Quidditch team or meant that most, not all, of the Ravenclaws ride Cleansweep Sevens. Or it could be due to the fact that they didn't know what Ravenclaw's line-up would be for the match, so they just generalized from what they had seen in previous games where Ravenclaw had played. Oliver Wood tells Harry that he just found out that Cho would be playing seeker, so how would the unnamed Gryffindor know who was playing, and more specifically, what broom they were riding; so it was most likely just a generalisation used just for the purpose of saying how much faster and better Harry's broom was than any Ravenclaw's brooms.
 * On page 101 of the American hardcover edition, Sybill Trelawney's name is misspelt as 'Sibyll'.
 * In the chapter "The Quidditch Final" when Katie Bell is taking her second penalty shot at the Slytherin vs. Gryffindor match, it says she put it past the Slytherin Seeker. The Seeker in Quidditch is the one who chases the Snitch, which, in the case of Slytherin, would be Draco Malfoy.
 * If Fred and George have had the Marauder's Map since first year, wouldn't they have noticed that Peter Pettigrew was in the male Gryffindor dormitories with Ron despite the fact they thought he was dead? -- J.K. Rowling later addressed this and stated that the duo only used the map for pranks and never noticed.
 * On page 261 Dumbledore is speaking to Harry and Hermione and tells them, "There is not a shred of proof to support Black’s story, except your word – and the word of two thirteen-year-old wizards will not convince anybody," but Hermione's birthday is in September, so she's 14 years old at this point. However this may not be considered a mistake as it could be explained simply by Dumbledore not remembering Hermione's birthday.
 * On page 285 Snape's name is misspelled as Snap in some versions of the U.S. Hardback Edition.
 * On page 117 of the UK paperback edition, where Harry is talking to Lupin about the Boggart and he mentions Voldemort, it says "The only person Harry had ever heard say the name aloud (apart from himself) was Professor Dumbledore". This isn't correct. In The Philosopher's Stone (The Keeper of Keys, pg 45, UK paperback), Hagrid says "Nah - can't spell it. All right - Voldemort".
 * However Harry may have deliberately excluded Hagrid as he does not use the name habitually.


 * In the early printings of the Scholastic edition, there are some books which were bound incorrectly, the spine and cover having been bound inverted. Upon opening, one notices that the text seems inverted, that the first page ones sees is really the last of the book.
 * For a supposedly spectator event, the staging of the Triwizard Tournament is remarkably unfriendly to spectators. Although the entire school turns out for every event, they can only see what happens in the first event, as the second occurs at the bottom of a lake and the third occurs within an impenetrable maze. It is hard to explain why no effort is made to give the audience a better view of the champions' adventures, as this is presumably the whole point of the tournament, and the justification for cancelling the entire season of Quidditch, a spectator sport that people can actually see.
 * In the book's opening chapter, there are repeated references to a planned murder, with lines such as "... one more death and our path to Harry Potter is clear" and "One more murder… my faithful servant at Hogwarts…" Yet, there is no indication that anyone else is murdered that was part of this plan. Bertha Jorkins was already dead, Frank Bryce was only a victim of circumstance, and Barty Crouch was originally kept alive and only killed because he had escaped and become a liability.
 * Voldemort's original plan may have been to kill Crouch Sr., and he was subsequently convinced otherwise by Wormtail or Crouch Jr., though if so, nothing was ever explained.
 * The death referenced may also refer to the planned future execution of Harry, as the "faithful servant at Hogwarts" is Barty Crouch Jr, who has taken Mad Eye Moody's appearance in order to capture Harry and sacrifice him to grant Voldemort a new body. At the time it is possible that Voldemort believed the "blood of the enemy" may have required Harry's death.
 * Also it is possible that Wormtail, not wanting to kill Harry, may have taken the blood from his arm to see if that would suffice for the resurrection spell.
 * This is the fault of the American version. The English version specifically said curse; not murder.
 * In the Scholastic Edition, in chapter eleven, in the 28th line of the Sorting Hat's new song, Hufflepuff is spelled as "Hufflepujf"
 * The narration mentions at one point that Harry "had only found out that Sirius was his godfather two months ago," when in fact it had been on the December Hogsmeade trip (in other words, eight months ago) when Harry overheard that Sirius was his godfather. It would be more accurate to say he "had only found out that Sirius was innocent two months ago."
 * When Harry is asking for his uncle's permission to go to the Quidditch World Cup and is describing Molly Weasley, he remembers that "nobody ever mentioned the name of [his] school [i.e. Hogwarts] in the Dursley household," but Harry himself did refer to Hogwarts by name nearly three years ago when he was asking for his uncle for a ride to King's Cross where the Hogwarts Express boards.
 * It was first mentioned, however, in the second book that Harry was forbidden to say any words related to magic while staying at the Dursley household, so it is possible that the Dursleys did not enforce this rule before then.
 * In "Back to the Burrow," Mr Weasley puts a pinch of Floo Powder into the fire for their group to return to the Burrow, but in book 2 everyone has to use Floo Powder individually to travel.
 * In "Back to the Burrow" Mr Weasley uses magic to deflect something that Vernon Dursley has thrown at him. In The Chamber of Secrets Harry receives a warning from the Ministry of Magic for using magic in the presence of muggles and also because he is under aged and away from Hogwarts, even though it was Dobby who used magic, not Harry. We later learn that this is because the Ministry doesn't know when a specific wizard uses magic, but rather, where it is used. Since the ministry knows Harry is the only wizard living in that general area, any magic performed is assumed to come from Harry. So why wouldn't Harry have received a letter after Mr Weasley performed magic in the Dursleys' home?
 * This might be due to the fact that Mr Weasley notified the Ministry that he was going there, so they couldn't just assume it was Harry.
 * Toward the end of the book when Voldemort puts the Imperius Curse on Harry, the book says "for the third time in his life," but toward the middle of the book it states Harry had the Imperius Curse put on him four times by Moody (Crouch, Jr.).
 * This, however, may have been a reference to the number of occasions and situations on which it was used rather than the exact number of times the curse was placed on him.
 * Harry's fourteenth birthday is on 31 July, 1994, but in his letter to Sirius at the beginning of the book he mentions that his cousin Dudley got really angry and chucked his PlayStation out of the window. The PlayStation was not released in Europe until 29 September, 1995. It could not have been imported either because the PlayStation was not released in Japan until 3 December, 1994.
 * However, it is possible that the Dursleys had signed up to be product tester, which would mean that they would receive the product (in this case, a PlayStation and a couple of games) months before the product was released.
 * It is also possible that Harry, due to his lack of interest in the materialism of the Dursleys, had confused the PlayStation with a similar console.
 * In what is now known as the "Wand Order Mistake," when Harry and Lord Voldemort's wands engage in Priori Incantatem while in the graveyard, James Potter exits the wand before Lily Potter. It has always been said that James died first as he was trying to give Lily and Harry time to flee, and since victims of Lord Voldemort were exiting the wand in reverse order of being killed, it was thought to have been a clue from J.K. Rowling.
 * Rowling later said that "late night writer's fatigue" was the reason, and the error has been fixed in later editions.
 * During the meeting between Fudge, Dumbledore, and Moody, Fudge is mistakenly referred to as both "Crouch" and also as his first name, "Cornelius."
 * In the European and Canadian versions of the book, Snape shouts, "Ten points from Hufflepuff, Fawcett! And ten points from Ravenclaw too, Stebbins!" Really, Fawcett is in Ravenclaw, and Stebbins is in Hufflepuff.
 * It may, of course, be that Snape, angry as he was, mixed up the two students' affiliation to the houses. The American version corrects this error, but in a deleted scene from the film, included in the uncut DVD, Fawcett is again the Hufflepuff and Stebbins the Ravenclaw.
 * In Quidditch Through the Ages, it says that the first ever World Cup was in 1473, with a match being held every four years. Yet, the World Cup in 1994 is number 422. There cannot be 422 world cups in 521 years with a four-year gap in between.
 * The period of years in between Quidditch matches in 1473 may have been slightly different, or the years may have been counted differently.
 * Before the third task starts, Molly Weasley reminisces about the gamekeeper before Hagrid, named Ogg, but in other books, it is implied that Hagrid has held that post since his expulsion from school in 1943. Since Hagrid's years as gamekeeper appear to overlap Molly's years at school, it seems unlikely that she knew a previous one.
 * It is possible that Hagrid simply started off as Ogg's assistant since he was still very young when he was expelled from Hogwarts since Hagrid himself states so in the first book.
 * When Fred and George bet on the Quidditch match with Bagman, it says Fred put the paper with their wages in his robes, but Fred was wearing Muggle clothing.
 * Barty Crouch Sr. mentions that his son got twelve O.W.L.s. Once again, this is inconsistent with the time-table clashes from Prisoner of Azkaban.
 * However, given Crouch Sr.'s damaged state of mind, he could have just been rambling insanely.
 * Another possibility is that it is not necessary to take a class in order to sit an O.W.L. This is logical since Remus Lupin mentions in Deathly Hallows that until Voldemort took over the Ministry, parents were allowed to educate their children at home, though virtually all British magical children attended Hogwarts. These students would presumably need the credentials given by an O.W.L. or an N.E.W.T. in order to obtain employment in many occupations. It is thus possible that a highly motivated student may be able to do "independent study" to achieve an O.W.L. or that a student was already so familiar with the Muggle world, for example, that they could easily pass an O.W.L. in Muggle Studies without attending classes.
 * In some printings, the spell to revive Krum and Crouch from unconsciousness is ennervate. The correct spelling is rennervate since ennervate would have the opposite effect (corrected in later editions).
 * The notice that says classes will end a half hour early for the arrival of the Durmstrang and Beauxbatons students states that 30 October, 1994 is a Friday. However, it was really a Sunday.
 * In the American paperback edition on page 148, Harry asks Ron if he can dump his things upstairs in his room. It then states, "'What's up Harry?' said Ron, the moment they had closed the door of the attic room behind them." In the Harry Potter and the Chamber of Secrets American paperback edition, on page 41, Ron says "... I'm right underneath the ghoul in the attic," referring to his room. Therefore the sentence in Goblet of Fire where he closes the attic door (to his room) does not make sense because that is where the ghoul resides.
 * Although with The Burrow being such a haphazard building, it is likely that it contains more than just one attic (an attic referring to a room beneath the eeves).
 * It is also possible that, due to the familiarity with the ghoul, the Weasleys managed to move it into a different room over the course of two school years. An example that supports this would be Ron's plan for the ghoul to take up residence in his room while he was away searching for horcruxes with Harry and Hermione.
 * Near the beginning of Chapter 14, Neville is assigned the task of disemboweling horned toads as punishment from Professor Snape. When Hermione teaches him the Scouring Charm to remove the filth from under his fingernails, they're referred to as "frog guts." However, the horned toad is actually a species of lizard, despite the name.
 * Since this was from Harry's point of view, however, he might not have known that fact and simply assumed the horned toad was a species of frog.
 * On page 594 in the first edition of the Canadian paperback and page 685 of the first edition American hardcover, when Dumbledore is questioning Barty Crouch, it says, "'How did your father subdue you?' said Dumbledore. 'The Imperius curse,' said Moody." Crouch is mistakenly referred to as Moody in this sentence because it was after Barty Couch had already returned to his original physical state (no longer an impersonation of Moody). Crouch had already been referred to by his real name earlier on that page, and the real Moody was lying stunned at the bottom of the trunk in his office.
 * The functionality of portkeys is inconsistent within the book. As introduced, portkeys are objects that will transport all those in contact with them to a predetermined place at a predetermined time. Subsequent books also use portkeys in this fashion. The Triwizard Cup portkey, however, transports the first person to come in contact with it, with no apparent regard for timing. In addition, it functions bidirectionally, transporting Harry and Cedric both to and from the Maze, whereas all other portkeys are treated as one-way enchantments.
 * It could be that Portkeys can have various different properties depending on the situation one is required for. If this were the case, it would be clear why Crouch Jr. had made the Portkey work regardless of the time, though it still seems odd that he would make it able to return Harry and Cedric to the Maze afterwards.
 * When the Weasleys, Hermione, and Harry arrive at the camp site for the Quidditch match, they have to use a map to find where they set up their camps. Yet Percy, Bill, and Charlie are able to find them right away after apparating in the woods. This is proved by Percy saying, "Just Apparated, Dad" as soon as they emerged from the woods. It seems impossible that the three could have found their camps that quick, unless Mr. Weasley had sent a message to his sons while collecting wood for their fire.
 * However it is said that their camping spot was located right next to the main path, so Percy, Bill and Charlie may just have got lucky.
 * It is also possible that the three eldest children did in fact go to find a map, and Percy was referring to them Apparating a short while ago.
 * Since Percy is a very proud person, who has just left Hogwarts, he may be simply bragging about his ability to apparate, while his younger siblings cannot.
 * In "The Hungarian Horntail" as Hagrid takes Harry to see the dragons he stops to pick up Madame Maxime, and Harry wonders to himself "Had Hagrid wanted to show him Madame Maxime? He could see her any old time he wanted...she wasn't exactly hard to miss....". However, as a meiotic reference to her great height, what he should have said is that Madame Maxime isn't exactly easy to miss.


 * In the American paperback edition, on page 161, when talking about the old Defence Against the Dark Arts teachers, Harry says, "One sacked, one dead, one's memory removed, and one locked in a trunk for nine months," referring to Lupin, Quirrell, Lockhart, and Moody respectively. However, according to the Prisoner of Azkaban American paperback edition, on page 423, Lupin did not get sacked — he resigned.
 * However, Harry could have interpreted Lupin's resignation as forced by Snape's revelation of his lycanthropy and therefore considered it the same as a sacking. Also, Fred does not mention either of his first two Defence Against the Dark Arts teachers, though it is possible that those two simply resigned and nothing particularly bad happened to them, thus no mention.
 * In Chapter 3, Tonks says she is an auror and she only qualified last year, but in Chapter 29, Professor McGonagall says that nobody has been taken on in the last three years.
 * However, she said she doesn't think anyone has been taken on so she might have just been mistaken.
 * We are also unaware whether there is any on the job training before qualification. Tonks may have been an auror for 2 years before becoming qualified.
 * In chapter 3, when the Advance Guard arrives at 4 Privet Drive to take Harry to 12 Grimmauld Place, Harry is already in trouble for casting a Patronus Charm in the presence of a Muggle. Yet, Tonks casts several spells (Lumos, a packing spell, Scourgify and Locomotor Trunk) and Moody casts a Disillusionment Charm on Harry while they are still at 4 Privet Drive. Lupin had clearly stated that they were not using the Floo Network because they were being watched (by the Ministry), so of course the Ministry was not aware of any Wizards that are allowed to do magic inside Harry's house. If the Ministry can detect magic, and Harry is the only known wizard in Little Whinging, then he should have either had another owl sent from the Improper Use of Magic Office or the uses of those spells should have been brought up at some point during his trial. Possible explanations include the fact that there were no Muggles present at the time, or that unlike Dobby, magic used by registered wizards does not activate the Trace, however this is inconsistent with the seventh book, when Harry is told they had to escape using Polyjuice Potion and physical transport because using any magic at all around Harry would alert the Ministry to his presence.
 * Also in chapter 3, Hedwig's cage has "droppings" (ie: poo). However, owls don't poop.  They regurgitate pellets.  I think this mistake is also present when Mr. Weasley tells Harry that they don't use owls in the Ministry of Magic because there were "droppings everywhere."
 * In chapter 19, Harry refers to the Dark Marks on the arms of the Death Eaters as scars when they are actually brandings.
 * In Chapter 2, after Harry tells Uncle Vernon about Voldemort and Uncle Vernon tells Harry to get out, Dumbledore sends Petunia a Howler which says "Remember my last, Petunia" referring to the letter Dumbledore had left on the doorstep with baby Harry. Later, in chapter 37, Dumbledore explains that Petunia "might need reminding of the pact she had sealed by taking [Harry]". However, in chapter 5 of the Chamber of Secrets, after Harry and Ron crash the Ford Anglia into the Whomping Willow, Dumbledore says he will "be writing to both [their] families tonight". This means that the Howler was referring to this letter about the flying car rather than the letter written by Dumbledore on the night of Lily and James' death as intended.
 * Sirius Black told Harry Potter that Bellatrix ran in the same crowd as Severus Snape while at Hogwarts, but her date of birth makes this impossible. Bellatrix would have attended Hogwarts from 1962 to 1969, or possibly 1963 to 1970, depending on which month in 1951 she was born in, whereas Snape began his schooling in 1971.
 * This may simply be an error since J. K. Rowling has admitted that maths is not her strong suit.
 * Another possible explanation could be that both Bellatrix and Snape went around with the same type of people, some of whom may have overlapped depending on age. For example, Lucius Malfoy was mentioned to be a prefect during Snape's Sorting in Deathly Hallows.
 * During Harry's hearing, Cornelius said that he blew up his aunt three years ago. However, the incident when Harry inflated Marjorie Dursley occurred almost exactly two years previously.
 * Although, this may be a mistake on Cornelius's part, and could show that he was not told accurate information, he had forgotten the date Harry blew up Aunt Marge, or that he was desperate enough to give away the wrong information; the second theory is most probable.
 * When Cho and Harry go to Hogsmeade together, she recalls their first match against each other "in the third year."  As she is a year older than him it should have been "in your third year."
 * She may have been referring to the third year they had been on their house Quidditch teams, Gryffindor and Ravenclaw, respectfully.
 * In chapter 9, Ron's prefect badge is described as scarlet and gold. Page 162 of the American paperback edition states, "A large P was superimposed on the Gryffindor lion. He had seen a badge just like this on Percy's chest on his very first day at Hogwarts." However, Percy's badge in Philosopher's Stone is described as silver with a P on it (later editions of the first book are fixed).
 * In chapter 9, Mrs Weasley says that everyone in the family has been a prefect, but Charlie was a Quidditch Captain.
 * It is possible that she was including Charlie since Hermione says in the sixth book that Quidditch Captains have equal status with prefects.
 * It is also possible that Charlie was a prefect in addition to being Quidditch Captain.
 * She also leaves out Fred and George, who ask, "What are we, next-door neighbours?"
 * It is mentioned that Remus, rather than James, was the male Gryffindor prefect in Harry's father's year. However, Hagrid mentions in Philosopher's Stone that James and Lily were Head Boy and Girl at Hogwarts. Since it is implied that the Head Boy and Girl are chosen from the prefects, this does not work.
 * Although it is implied that the Head Boy and Girl are chosen from the prefects, it is never explicitly stated, so James could have been Head Boy without being a prefect.
 * If James had been Quidditch Captain, he could have become Head Boy through that, as Quidditch Captains have equal status with prefects.
 * Hagrid could simply have been mistaken in thinking that James had been Head Boy.
 * In chapter 12, Ron says "That's the worst Monday I've ever seen" in reference to his and Harry's schedule for the first day of classes. As school commences with the feast on 1 September, this first day of classes would fall on 2 September. However, at the end of Chapter 6, it is written that the night before Harry's trial is a Wednesday. Thus, Harry's trial, which takes place on 12 August, falls on a Thursday. So 2 September would actually have to be a Thursday, as well. (And in 1995, 12 August and 2 September were actually Saturdays.)
 * During the meeting in the Hog's Head, Dennis Creevey shows up for the meeting. However, in Harry's fifth year, Dennis would only be a second year, and therefore would not be able to visit Hogsmeade.
 * He could, however, have sneaked out of the school with his brother, although what with Filch guarding the entrance and the Creeveys not having been notified of any secret passageways, this seems unlikely (unless they discovered one by chance).
 * However, Fred and George were both aware of who Hermione was intending to recruit (having been recruited themselves) and could have shown him one of the secret passageways.
 * After Harry has seen his father bully Snape in a memory, he remembers Professor McGonagall saying that James and Sirius were troublemakers at school but no more so than the Weasley twins. This is incorrect. She said in Prisoner of Azkaban that she had no memory of anyone else being so troublesome, and Hagrid said Fred and George could have given them a run for their money.
 * One possible explanation is that McGonagall's earlier statement was a hyperbole. Regardless, it was still not her who said that they were like Fred and George. (It actually just states in the book that Harry imagined them as precursors of Fred and George, so McGonagall didn't really imply it.)
 * In Chapter 30 of the American hardcover edition, Harry and Hermione find seats in the topmost row at the Quidditch match, but when Hagrid comes to tell them about Grawp, he squeezes his way through the row behind them. If Harry and Hermione are in the topmost row, there cannot be a row behind them (corrected in later editions).
 * It states that after watching Ginny play it was Harry's first time watching Quidditch in the stands but Harry would have watched all the other matches that didn't involve Gryffindor (all matches since his first year at Hogwarts).
 * In the chapter "O.W.L.s," it is stated that the practical Astronomy O.W.L. began at 11:00 pm, and Harry observed Venus one hour later (12:00 am). However, Venus can only be observed shortly before sunrise or shortly after sunset; it can never be seen at midnight. Harry was also filling in his star chart with the stars in the constellation of Orion. However, Orion would have set before 11:00pm on that date. Also, Hogwarts is somewhere in the Highlands of Scotland (according to the movie of Prisoner of Azkaban, it's near Dufftown in Moray); at that latitude, in the two months either side of June 21, the sky never gets completely dark. There is at least one website showing that sunset on the day of the examination would be about 11pm — the time the examination is supposed to start. It would be unlikely to be dark enough to observe any but the brightest stars.
 * The telescopes may be charmed to show every star regardless.
 * During the Ordinary Wizarding Level examinations, Hannah Abbott was doing her Transfiguration practical at the same time as Harry Potter. This is odd since the examinations are done by surname, meaning that Hannah should have finished a while ago, unless she was a rather incompetent student, or unless she had a fit of anxiety in the middle of the examination and had to be calmed, which could take time, and then resume the exam when Harry started his. (it is implied that Hannah was given a calming draught).
 * When Snape calls Voldemort "the Dark Lord," Harry makes an accusatory statement, claiming to have only heard the Death Eaters call him that, but in Prisoner of Azkban, Professor Trelawney, who has never been suspected of being a Death Eater, called him that.
 * However, Trelawney's use of the title "the Dark Lord" was used largely for poetic effect rather than out of loyalty. Even earlier, in Chamber of Secrets, Ginny Weasley wrote a poem that referred to Voldemort as "the Dark Lord." Another year before that, in Philosopher's Stone, Ollivander also called him "the Dark Lord" as he talked about his past achievements.
 * It is possible that Harry forgot these examples due to his anger at Snape, or ignored them in favour of accusing Snape of being a Death Eater.
 * In the UK paperback celebratory edition, in the chapter "Beyond the Veil", the first line of the sentence which runs from page 710 to 711 is repeated, first on the bottom of page 710 and again at the top of page 711. The line reads, "Harry saw the look of mingled fear and surprise on his god-".
 * Harry mentions that he gave the interview on the last Hogsmeade weekend. He did it on 14 February, which was a Wednesday in 1996.
 * During the trip of Ministry of Magic with his friends, it was left out why there was no Order members there on watch duty, as in the beginning of the book, they mentioned about constantly being on watch duty, day and night.


 * On page 101 of the earliest printings, and in the original audiobook editions of Harry Potter and the Half-Blood Prince, Ron mentions that Hermione got ten Outstandings and one Exceeds Expectations for Defence Against the Dark Arts, giving her a total of eleven O.W.L.s. However, since she dropped both Divination and Muggle Studies in her third year, she could not possibly have got eleven O.W.L.s since she was only doing ten subjects — Transfiguration, Charms, Herbology, Potions, Defence Against the Dark Arts, History of Magic, Astronomy, Care of Magical Creatures, Arithmancy, and Ancient Runes (This error was corrected in later editions, in which Ron instead says nine Outstandings and one Exceeds Expectations.)
 * In the sixth book, a comma is placed instead of a full stop at the end of a paragraph that ended in `Snape`. The next paragraph has nothing to do with the previous paragraph, so it shouldn't end in a comma.
 * Harry's sixteenth birthday happens on 31 July, 1996. Looking at any full moon calendar, in that year the full moon was on 30 July. Remus Lupin was recovering from the after effects of his werewolf transformation and it is therefore difficult for him to go to birthday parties. It is said, though, that he was looking "gaunt and grim," so he might have transformed the night before. At Christmas, however, it is stated that Remus Lupin comes over to the Burrow and even uses Bill's bedroom. 24 December, 1996, is the exact date of a full moon and, since Lupin was already living with the werewolves, there was little chance he could get the seven days worth of doses of Wolfsbane potion required to make him harmless. Therefore, on that night he would have been a dangerous transformed werewolf.
 * On page 126 in Chapter Seven on some American Editions it says Chapter One.
 * In the chapter "The Secret Riddle," it says, "... Dumbledore entered the room... It was a small bare room with nothing in it except an old wardrobe and an iron bedstead." A couple paragraphs later, "Dumbledore drew up the hard wooden chair beside Riddle, so that the pair of them looked rather like a hospital patient and visitor."
 * This may have been a pun, since with Dumbledore especially, the term "draw up a chair" is used literally several times when Dumbledore magically conjures a chair by drawing it. This may have been a covert way of implying this, especially considering Riddle may have missed this action by Dumbledore.
 * On page 342 of the American version, there is a punctuation error when Molly Weasley says "Have a little purkey, or some tooding...I mean-" to Rufus Scrimgeour. There are no quotation marks in front of the "have" in the statement. This mistake is corrected in later printings.
 * Harry saw the year of the book of the Half Blood Prince was 50 years old which "his father could not have been the prince" but Snape was in the same year as James which meant he could not have been the prince either UNLESS he bought the book years after the book was published, then James could have also potentially been the owner of the book.
 * At the start of term feast, Dumbledore tells Quidditch hopefuls to put their names down for selection. For the Gryffindor House, at least, there had never been trials before. Katie Bell also warns Harry against just "picking the old faces," but this had automatically been the case for the five years preceding Harry's sixth year. It is likely Wood was happy with the team for three years, and there was no reason to hold a trial with no spots becoming vacant up until he left. There was no need to hold trials in Harry's fourth year. Angelina held trials in Harry's fifth year, but only for the parts that needed to be filled in; this could mean that Angelina was happy with the remaining players on the team.
 * This may also be a sign of Harry simply bowing to pressure since he could have chosen to allow those that were already on the team to continue playing but instead chose to take Katie's advice.
 * It is also possible that a new team is chosen only if there's a new captain.
 * Lastly, it could be that Katie was referring to the bad players from the year previous, and simply wanted to replace the players whom had weighed down the team.


 * At the beginning of chapter 2, it says that Harry cannot use magic due to not coming of age for another four days. In chapter six, days have passed since the Battle over Little Whinging, but Harry states the Trace will break in four days.
 * In chapter 20 Ron explains that Voldemort had placed a Taboo on his name so that if anyone said it he would be able to find that person, and explains that was how the Death Eaters had found them on Tottenham Court Road—Hermione used the name once, while they were sitting in the cafe. However, the trio used Voldemort's name several times while they stayed at Number Twelve Grimmauld Place, and the Death Eaters did not manage to find them there until Hermione accidentally brought one back with them, after Disapparating from the Ministry of Magic. The charms placed on 12 Grimmauld Place which make the location impossible to enter unless invited but the jinx placed on Voldemort's name is specifically said to "break protective enchantments." However, it is possible that it may not be able to break the Fidelius Charm.
 * In chapter 20, when Harry, Ron, and Hermione are at the front door of Xenophilius Lovegood's house, Hermione says to Harry, "You better take off the Invisibility Cloak, Harry, it's you Mr. Lovegood wants to help," and he handed her his Cloak to place in the beaded bag, but after the Death Eaters arrive Hermione says, "Harry give me the Invisibility Cloak, Ron, you're going to put it on.
 * In chapter 23, Narcissa says that Draco "is home for his Easter holidays." In Chapter 24, the sun is described as rising at Shell Cottage in Cornwall, but shortly after this, Harry sees into Voldemort's mind at Hogsmeade, where it is "still dark, because it was so much farther north." However (in the absence of a significant longitudinal difference, as in this case), this sort of thing is only true between the Autumnal and Vernal Equinoxes; Easter occurs after the Vernal Equinox  (e.g. on 12 April in 1998), in the other half of the year when sunrise is earlier the further north one goes, thus it should not have been dark at Hogwarts when Harry saw into Voldemort's mind.
 * In chapter 25, "Shell Cottage", Lupin says that Bill is the Cottage's Secret Keeper, yet Ron was able to tell Dobby about where the cottage was when escaping from Malfoy Manor. However, the Fidelius Charm may not have been placed upon the cottage at the time, or Ron was already aware of the house previously at an unknown point, or it might be that the Fidelius Charm does not apply to House-Elves; several times in the series, elves are able to apparate to places witches and wizards cannot.
 * In chapter 26, when Harry, Ron, and Hermione break into the Lestranges' vault at Gringotts in Chapter 26, Hermione performs a whispered Levicorpus spell on Harry so that he can reach the Horcrux without touching the other contents of the vault. The Levicorpus spell, however, is most effective as a nonverbal spell, which Hermione ought to have been capable of given her talent.
 * Of course, this may be because Hermione was intending on warning Harry on what she was doing, rather than catching him off-guard.
 * In chapter 27 Harry drips dittany through the singed hole in his jeans to the angry burn beneath, but he has just got changed, so he shouldn't be wearing his singed jeans anymore. Not much later in this chapter, Harry returns from an excursion into Voldemort's mind, and is "vaguely surprised" that he is "still wet to his skin", which he should not be since he had got changed into dry robes earlier. Harry may have been so effected by this excursion that he woke up in a cold sweat, however this is unlikely, since Harry has had a lot of experience seeing Voldemort's thoughts by this point, and because the statement implies that Harry's clothing is still soaked.
 * When Harry is viewing Snape's memories, he witnesses a scene on Platform 9¾ between his mother and Aunt Petunia, in which Petunia tugs her hand out of Lily's grasp. Her hand is then back in Lily's, and she tugs it out again.
 * Lily may have grabbed her hand again against her will.
 * Similarly, if Petunia had been on Platform 9¾ before, she should have known where it was and been able to tell Vernon.
 * Given her nature, it is very likely that she simply chose not to out of spite.
 * In chapter 33, "The Prince's Tale", we see Dumbledore asking Snape to cast a Confundus Charm on Mundungus Fletcher, but since Voldemort took over the Ministry after the chase, Snape shouldn't have been able to enter Hogwarts and go to the Headmaster's office. Unless Dumbledore showed him a way to enter unnoticed, had allowed him access, or something similar.
 * It is also possible he used the Vanishing Cabinet in Borgin & Burkes to enter the Room of Requirement at night and thus sneak to the Headmaster's Office, or used the same passage through the Room of Requirement that the DA later used..
 * Alternatively, he might have entered at day time and used a Disillusionment Charm in order to render himself almost invisible and spy on his former colleagues in order to find out what the password for the office is.
 * Ron went to the shell cottage after he left Harry, but how could he find it if Bill was secret keeper? Ron apparated right into some snatchers, got out, and went where?
 * Colin Creevey snuck back in order to join the fight however he was a muggle-born and should not have been at Hogwarts when the Carros where in charge.