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.

Harry Potter and the Philosopher's Stone

 * Much of the first chapter talks about wizards celebrating the fall of Voldemort all day long, yet Harry and his parents were attacked at night; unless it took Hagrid 24 hours to get Harry from his parents' ruined house in Godric's Hollow to Little Whinging, this seemingly does not make sense. A possible explanation, however, is that it does not say when Hagrid pulled him from the house. It could have been a full day until they could respond, or he could have kept Harry with him until night, when he could easily fly under cover of darkness to Little Whinging. He could have also gone to get baby Harry checked over to see if he had been harmed. The day, however, is 1 November, the day after Voldemort's downfall, so the celebrations are happening after his downfall the evening prior. We are also told later on in the Prisoner of Azkaban that Sirius was present for some time before Harry was given to Hagrid, so this may account for some more time.
 * Dumbledore says that he passes through several parties to get to Little Whinging, however he got there by Apparition, which is a direct form of transport, although he could have apparated several times to go to many parties.
 * On page 17 of the Bloomsbury edition, Hagrid says, "I'll be takin' Sirius his bike back." But in the third book, it is made clear that Sirius told Hagrid to keep the bike because he (Sirius) didn't need it anymore. (This was changed to "I'd best get this bike away" in later editions.)
 * The beginning of the story takes place on a "dull grey Tuesday." In 1981, however, Oct 31 was on a Saturday, placing the celebrations on Nov 3. This makes what the TV weatherman wrong as he admonished people that Bonfire Night was the next week. In fact, it would have been in two days. The two days may in fact have crossed the boundary of that week, making the weatherman correct.
 * The zoo trip took place on Saturday, but Dudley's eleventh birthday (June 23, 1991) actually was on a Sunday.
 * The boa constrictor at the Zoo winked at Harry during their conversation. As snakes don't have eyelids, it is impossible for them to blink, for their eyes are protected by transparent scales.
 * Harry's 11th birthday, 31 July, was a Tuesday. But in 1991 in the real world, that date was a Wednesday.
 * On page 56, the sentence "Uncle Vernon certainly seemed to have got back his courage" is incorrect. It should be "Uncle Vernon certainly seemed to have gotten back his courage."
 * When Harry and Hagrid are leaving the little shack out in the middle of the sea, they used the rowboat that the Dursleys had borrowed in order to leave the rock. This leaves the Dursleys with no way to get back to shore, yet there is no mention of retrieving them, and it is implied Harry goes straight back to Privet Drive after his shopping at 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. (Rectified 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, and only when the weather has been relatively warm. Once you get below the level where the temperature depends heavily on the seasons at the surface, the geothermal gradient takes over, and it gets hotter as you go deeper. In the world's deepest mines, the miners can only work for a short time before being brought back up, because of the risk of heat stroke.
 * Much importance is placed on the theme of wandlore and wand ownership throughout the series, so it seems odd in retrospect that, during Harry's first meeting with Draco Malfoy in Madam Malkin's, he should mention that his mother is away looking at wands. Indeed, it is just as odd to hear that Ron has been equipped with Charlie's old wand. It is mentioned, however, in Deathly Hallows and on Pottermore that there are old superstitions about wands and what kind of people are chosen by certain wands and so it's possible that Narcissa Malfoy had certain beliefs along those lines. She may also be looking at wands for Draco. As for Ron, his parents probably could not afford to buy him a new wand at the time. This was probably meant to show that his family was not very well off in terms of wealth. Even though it is better to have your own wand, a person can still do magic with another person's wand.
 * 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 when this conversation takes place, Hagrid (along with the rest of the wizarding world) would have been under the impression that Sirius Black, a Gryffindor, was responsible for betraying Harry's parents. This, however, is potentially simply an instance of the literary device of hyperbole ("the use of a statement to evoke strong feelings or create a strong impression but not meant to be taken literally"), rather than ever being intended as an accurate historical statement, even if one ignores the fact that there are known to be dark wizards from other schools, such as Grindelwald. This could also mean that Hagrid was referring to the family in Sirius's case, seeing as the Black family had all been in Slytherin with the exception of Sirius.
 * 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. The sixth movie has shown that Little Whinging is very likely within a few miles of Surbiton (on the lines from Waterloo), and hence not within several miles of any station served by trains from Paddington.
 * On his search for Platform 9¾, Harry overhears Molly Weasley asking her children "What platform was it again?" to which Ginny replies "9&frac34;". Considering that she must have been on that platform about 20 times, it seems odd that she can't remember its number but it is likely that she was just checking her children's knowledge and memory, or trying to make Ginny feel included.
 * 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 is only in the US edition - in the UK edition; Dean Thomas's name is omitted (rectified in later editions).
 * When Harry first met Nicholas de Mimsy-Porpington, the ghost says he had not eaten in nearly 400 years. Yet he had, at the time, already been dead for 99 years longer than that. (rectified in later editions).
 * Harry buys the book One Thousand Magical Herbs and Fungi, but later, he looks up "dittany" in One Hundred Magical Herbs and Fungi (rectified in later editions).
 * During the Hallowe'en feast, when the troll is let in, Professor Dumbledore sends the students to their dormitories. However, the troll is said to be in the dungeons, and that is also where the Slytherin dormitories are, meaning 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 prefects/teachers looking for missing students.
 * 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; indeed, 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.
 * 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, however, possible he meant "next to him" to mean "on the same side of the king and queen."
 * At the beginning of the chess match, Ron is the knight in the giant chess set. However, it later says that he had to "move ahead one" so the Queen could take him, opening the path for Harry to checkmate the king. If he were a knight, he could only move 1) ahead one, sideways two, or 2) ahead two, sideways one, like an "L." It is possible this meant "one" as in a single move, but it is still an odd construction regardless.
 * On Chapter 14, the night Charlie's letter shows up at Hogwarts is stated to be Wednesday at midnight. However in the chapter "The Potions Master", it is mentioned that Astronomy classes take place every Wednesday at midnight. Therefore, they are presumably skipping class, and the other students should not be asleep (this is corrected to Saturday in later editions).
 * The art for the cover of the UK edition depicts Harry at platform 9 3/4 wearing a Gryffindor scarf, however, Harry hasn't been sorted yet, thus not knowing he will be sorted into Gryffindor house.
 * The list of school supplies that Harry receives doesn't include potion ingredients, although Harry later buys them.
 * Dumbledore said that his and Hermione's owl 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.
 * 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 be Sally-Anne Perks, but because many students were pulled out of the school that year, it is possible Sally-Anne was cut out.
 * On Harry's list, he has to buy "Fantastic Beasts And Where To Find Them", but he doesn't start his care of magical creatures class until 3rd year. However, the book could be used for other classes (Though this is unlikely)
 * 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 is describing his spell too narrowly, Quirrell should have been able to retrieve the stone.
 * If the fact that Quirrell wants to give the stone to someone who will use it is enough to block his access, then this protection also renders the stone essentially irretrievable to Dumbledore or the Flamels.
 * In the last chapter, 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.

Harry Potter and the Chamber of Secrets

 * 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.
 * The Hogwarts' equipment list mentions the Lockhart book "Wanderings with Werewolves," but later, Lockhart refers to the book as "Weekend with a Werewolf" (fixed in later editions).
 * 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.
 * When Harry and Ron arrive at Hogwarts in Ron's father's flying Ford Anglia (a scene which takes place on September 1, 1992), they are told that they had been seen by Muggles in various situations, including flying past the "Post Office Tower." Quite apart from the fact that a change of ownership means that the building in question has been called the "BT Tower" since 1984, said building is about a mile southwest of King's Cross, so to be passing it, Harry and Ron had to be going in entirely the wrong direction to be following the Hogwarts Express, or even the tracks generally.
 * After Harry, Ron, Fred, and George arrive at the Burrow after getting Harry, Mrs. Weasley shouts at Fred first. "You will not," then she says, "and you two," glaring at Ron and Fred. It should say George since she has already yelled at Fred (fixed in later editions).
 * Dumbledore tells Harry at the end that Lord Voldemort is the last remaining ancestor of Salazar Slytherin. But ancestors are in the past — he means Voldemort is the last descendant of Salazar Slytherin (corrected in later printings).
 * Hermione says 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 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, and this was one of the few edits made for the North American release, despite the criticism on edits for Philosopher's Stone.
 * When Harry is viewing the memory from Tom Riddle's diary, Tom Riddle says to Hagrid at one point, "The dead girl's parents will be here tomorrow." It is stated elsewhere in the book that Myrtle is Muggle-born. Muggles cannot enter Hogwarts. It is possible, however, that an exception was made for them — that the Muggle-Repelling Charm could be lifted a day or two for this reason or that the Headmaster would apparate them to Hogwarts. Considering that the Grangers were able to enter Diagon Alley without any problems when normal Muggles could not see The Leaky Cauldron, this theory is somewhat strengthened. Although Myrtle's parents and Hermione's could be able to go to Hogwarts and see The Leaky Cauldron because their child is a witch.
 * When Harry and Ron go to Aragog, they feed Fang treacle toffee to glue his teeth together. Later, however, "Fang suddenly let loose a great bark." It is possible that the treacle toffee dissolves after a certain period of time.
 * February 14, 1993, was a Sunday in the real world, yet in the book the students had their classes on that day because Harry got his "musical message" when he was walking upstairs for Charms.
 * 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.
 * They might, however, hear pipes hissing but subconsciously ignore it, dismissing it as creaking wood, an air current going through the pipes, or other animals such as mice.
 * Alternatively, it could be that the snake is simply making a very quiet hissing. The basilisk's voice is implied to be a whisper and Harry, being the only one to hear English words, would be the only one to notice anything. Another explanation could be that Harry could originally not tell where the voice was coming from, so it must have been quiet (this strengthens the other theory).
 * The first day of term is always September 1, and the next day is always a Monday. This is impossible, due to the fact that the year does not have 364 (divisible by 7) days.
 * On page 283 of some copies of the US paperback edition, the word professor is misspelt as professorr.
 * When Colin Creevey is brought to the hospital wing after being Petrified and Professor McGonagall asks if he might have managed to take a picture of his attacker, Dumbledore opens the back of Colin's camera without attempting to rewind the film. Even if Colin had managed to photograph the basilisk, the undeveloped film inside would have been fogged immediately when Dumbledore looked at it, thus ruining the image.
 * However it is mentioned earlier in the book that Colin was using an ordinary Muggle camera, so Dumbledore might not have had any idea of how to handle one.
 * Although it's stated that the Basilisk was able to move through the castle's pipes, it is unknown how something as large as the Basilisk would be able to fit through the plumbing, or where it was exiting into the castle undetected in order to attack students.

Harry Potter and the Prisoner of Azkaban

 * When Harry, Ron and Hermione first find a carriage and 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 (the one with Lupin). Since the trio stowed their luggage earlier, there would be no need for them to go looking for an empty compartment. The book implies they find a different compartment than the one they stored their luggage in. 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.
 * Hermione's Time-Turner is a major secret that supposedly took a lot of persuasion to obtain. However, it should not be such a secret and 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, it is entirely possible 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. In addition as Hermione was the only Gryffindor said to be taking the classes she could be the only person with this problem as she would be attending classes with other Houses instead of her own. But Ron tells Harry and Hermione in book 1 that Percy got his 12 O.W.Ls, this is not possible unless he used the time turner as Hermione gets only 10 O.W.Ls minus Divination and Muggle Studies.
 * The first chapter says that A History of Magic was written by Adalbert Waffling, rather than Bathilda Bagshot (rectified in later editions).
 * Given that Remus Lupin had a transformation sometime after Halloween but before the following weekend (the day of the Gryffindor-Hufflepuff match), the full moon would have had to be sometime in the first week of November, probably the 4th or 5 November since he was absent from class on the Friday before the match. Since the full moon occurs every 29–30 days, by that time frame the December and January full moons would also have occurred sometime around the 3rd-5th of those months. Therefore a full moon during the Christmas Eve to Boxing day 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. It is not specified in the books, however, how long a werewolf might suffer ill effects from transformation, and how much variation there is between months. Therefore, he may simply be suffering pre-transformation effects and, as the Wolfsbane potion must be taken in the week preceding the full moon, then the next full moon may be as late as New Year's Eve. Notably, Snape says in the Shrieking Shack that Lupin had not taken his potion that night, but it is explained that he must drink the Wolfsbane Potion the week before transforming — not on the night itself — so missing the potion shouldn't have affected him within the space of a few hours.
 * During his substituition 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.
 * Snape may have not counted the first time.
 * 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.
 * When Harry is buying his books at Flourish and Blotts, it mentions that he is being served by the manager, but when Harry was staring at the book Death Omens, it says that he is being served by the assistant of the store.
 * Though it is possible that the manager was simply the assistant to the shop owner.
 * 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.
 * Yet it is possible that they sneaked into an office and retrieved the information of their results or were assuming that they got low marks.
 * 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 with Harry in the 1992-1993 school year, but the other time Harry went into the forest was for detention in the 1991-1992 school year, and Ron was not there (he was in the film but not in the book).
 * The chapter "Professor Trelawney's Prediction" takes place in June 1994. In the beginning of that chapter, Harry Potter reads a note from Hagrid and says "Buckbeak's appeal — it's set for the sixth." The night after the Hippogriff's appeal, Remus Lupin is forced into a werewolf form by the full moon, which means that this happened on the night of 6 June, 1994. The web page HP-Lexicon.org also backs this up. However, a brief look at lunar phase calendar sites such as 1994 and lunar phases in 1994 definitely shows 9 June, 1994, as a new-moon night, which means that a full moon could not possibly rise three days earlier.
 * 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 Two Sixty.
 * 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.
 * 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 by Dumbledore being the headmaster of a school with several hundred students attending at any one time, he has held the positions for multiple years and was Transfiguration professor beforehand and had been employed by the school in excess of fifty years, he cannot be expected to remember personal details such as dates of birth for each and every student.
 * On page 285 Snape's name is mispelled as Snap in some versions of the U.S. Hardback Edition.

Harry Potter and the Goblet of Fire

 * 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.
 * 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 Dursley's 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 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 4 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 moved 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 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.
 * 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.

Harry Potter and the Order of the Phoenix

 * 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.
 * 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 refering 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?"
 * 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-".
 * During the Gryffindor-Hufflepuff Quidditch match, Ron is said to fail to save fourteen goal attempts from Hufflepuff. However, Hufflepuff's final score is two hundred and thirty. The only ways to gain points in Quidditch is goal-scoring and catching the Golden Snitch, the latter of which was done by Gryffindor. Therefore, Hufflepuff must have made twenty-three goals, not fourteen.
 * However, it is possible to interpret the wording of the passage as Harry claiming that Ron's fourteenth save attempt was particularly poor as compared to the others, not that there were only fourteen total.

Harry Potter and the Half-Blood Prince

 * 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.)
 * 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 say 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.
 * 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 6th 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.

Harry Potter and the Deathly Hallows

 * At the beginning of chapter two, 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 accidently 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 on 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.
 * 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.
 * When Harry, Ron, and Hermione break into the Lestrange's 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.
 * This was probably done so that the reader knew what Hermione was doing.
 * 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.
 * Near the end of chapter thirty, it says that the only people left in the Room of Requirement are Harry, Ginny, Mrs. Weasley, Lupin, Fred, George, Bill, and Fleur, but on the next page Percy (who had recently entered the room) apologises to Mr. Weasley, but Mr. Weasley had left the room along with all the other people.  Though this could have been a typo, and "Mr." should have been "Mrs."
 * 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 the chapter "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.
 * In the seventh book, there was a typo of "Dumblefore" instead of "Dumbledore." (fixed in later editions)
 * In the part where Harry, Ron, and Hermione are camping out, they cannot manage to find good food and Ron is always complaining. However, Dean Thomas, Ted Tonks, Dirk Cresswell, Griphook and Gornuk are able to summon a salmon from the same river that Harry, Ron, and Hermione are near. The book says the "delicious smell of baking salmon wafted tantalisingly in their direction." There is little reason as to why Harry, Ron, and Hermione would not be able to obtain good food very easily this way.
 * The answer is simple - it never occurred to them. Even Hermione is not omniscient.
 * In the chapter Shell Cottage, Lupin says that Bill is the Cottage's Secret Keeper, yet Ron was able to tell Dobby about the cottage is when escaping from Malfoy Manor. However, the Fidelius Charm may not have been placed upon the cottage at the time, 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.