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

 * In this book, Harry states his 11th birthday, 31 July, was a Tuesday. But in 1991, that date was a Wednesday.
 * While being sorted into houses, Harry looks up at the Sorting Stool, and there are only three people left to be sorted. Professor McGonagall then calls out the names of four more students. However, it is possible that Harry missed out on one. (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". The title changes. However, it is possible that these could be two different books e.g. the latter could be an older edition of the former. (Rectified in later editions)
 * The book mentions that the Boa Constrictor at the zoo winked at Harry. Snakes do not have eyelids at all, so it is impossible for them to wink.
 * 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).
 * Much of the first chapter talks about wizards celebrating the fall of Voldemort all day long, yet Harry's family was attacked at nighttime. Unless it took Hagrid 24 hours to get Harry from his parents' ruined house to Little Whinging, this does not make sense.
 * It states in the Hogwarts letter that you can bring an owl, toad, or cat. Ron brings his rat Scabbers. These guidelines were probably fairly lenient. Lee Jordan also brings a tarantula and later Ginny a Pygmy Puff. This should not be seen as a mistake, just a leniency in school rules.
 * On page 221 of the American hardcover, the third paragraph begins "...wished him luck outside the locker rooms the next afternoon." But Harry couldn't have just been getting to the locker room in the afternoon; Quidditch matches take place in the late morning: the school teams are usually dressed and already on the pitch by 11:00 am.
 * In chapter 14, some of Charlie Weasley's friends arrive at Hogwarts on broomsticks to help smuggle Norbert the dragon out of the school. But in Prisoner of Azkaban, Hermione, who knows Hogwarts, A History by heart, mentions there are enchantments around Hogwarts that prevent people from entering the school by stealth, like flying over the castle walls.
 * However it is possible that Albus Dumbledore secretly knew about this event and then took off the enchantment for some moments.
 * SPOILER: Towards the end of the book, Dumbledore 'flew off to London' when he received the urgent owl from the Ministry and would be gone until the next day. Why wouldn't he have taken a much more efficient method of transport such as apparition or the Floo network?
 * However, "flew off" may have simply been a figure of speech - as it was an urgent owl, he may have left speedily, warranting the comment "flew off to London."
 * As we now know that much importance is placed on the theme of wandlore and wand ownership, it seems ridiculous that, during his first meeting with Malfoy in Madame Malkin's, Draco should mention that his mother is away looking at wands. Indeed, it's just as odd to hear that Ron has been equipped with Charlie's old wand when it is unusual for a wand to wear out and need replacing.
 * On page 205 of the UK PS, 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.
 * 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 was a knight, he could only move 1) ahead one, sideways two, or 2) ahead two, sideways one, like an "L."
 * SPOILER: When confronting Quirrell at the end, Quirrell frequently, and rather flippantly, refers to Lord Voldemort by name when we know from later books that He-Who-Must-Not-Be-Named would not allow such insolent behaviour from any of his followers and Quirrell would refer to him, most likely, as The Dark Lord.
 * On page 153 of the American Hardback Edition, Fred says that Gryffindor hasn't won the Quidditch Cup since Charlie left. According to JKR, Charlie is three years older than Percy who is in his 5th year meaning that Charlie would've left the year before. So basically Fred is saying that Gryffindor hasn't won since last year.
 * 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.
 * When Harry is talking to Hagrid about his first meeting with Draco Malfoy, the giant tells Harry that being in Hufflepuff would be better than Slytherin and explains 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 student at Hogwarts) was responsible for betraying Harry's parents. And even though he didn't know it at the time, he wasn't accurate anyway, seeing as Pettigrew was in Gryffindor.
 * 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. If Harry and Hagrid used the rowboat, how would the Dursleys' have gotten back?
 * On page 53 of the American paperback, when Aunt Petunia is talking about her sister Lily, she says, "Oh, she got a letter just like that and disappeared off to that -- that school -- and came home every vacation with her pockets full of frog spawn, turning teacups into rats." At the end of the school year at Hogwarts, "notes were handed out to all students, warning them not to use magic over the holidays" (pg 307) meaning that little Lily Evans would have been in trouble with the Ministry of Magic for "turning teacups into rats" during vacation.

Harry Potter and the Chamber of Secrets

 * The Hogwarts' equipment list mentions the Lockhart book "Wanderings With Werewolves," but later, Lockhart refers to the book as "Weekend With a Werewolf". This, however, could be a particular chapter within that book.
 * 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, for she has already yelled at Fred. (Fixed in later editions)
 * On page 83 it says in the last paragraph 'Fred Weasley's head drooped right onto Alicia Spinnet's shoulder and he began to snore.' but in the next paragraph it says ' 'I've got a question, Oliver,' said George, who had woken with a start. It could have been, however, that both twins had fallen asleep.
 * 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, however, 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.)
 * Harry, Ron and Hermione go to Nearly Headless Nick's 500th deathday party. However, in the first book, at the start of term feast, he says he has not eaten for nearly 400 years, which would imply that this is how long he has been dead.
 * There is a scene after Ron walks out of Moaning Myrtle's bathroom where Percy takes five points from Gryffindor. However, in Order of the Phoenix, at the beginning of the chapter "Snape's Worst Memory," it is clearly stated by Ernie Macmillan and Ron that prefects cannot dock points. This is corrected in later editions as Ernie Macmillan says that prefects cannot dock points from each other.
 * 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. However, it is possible that an exception was made for them (the Muggle-Repelling Charm could be lifted a day or two, for this reason).
 * 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. However, this may be because they mistake the hissing for the movement of water in the pipes.

Harry Potter and the Prisoner of Azkaban
On page 74, it states that "Harry, Ron, and Hermione set off down the corridor, looking for an empty compartment, but all were full except for the one at the very end of the train. This one had only one occupant, a man sitting fast asleep next to the window." They had just put their belongings into an empty compartment, so they didn't need to look for another. On page 76, it states, "A faint, tinny sort of whistle was coming from somewhere. They looked all around the compartment. 'It's coming from your trunk, Harry,' said Ron, standing up and reaching into the luggage rack." Their trunks couldn't have been in that compartment as they had stowed them in another one.
 * The first chapter says that A History of Magic was written by Adalbert Waffling, rather than Bathilda Bagshot.
 * 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 5th of November as he was absent from class on the Friday before the match. As 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 as Snape is mentioned as having brewed his potion for him.
 * 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.
 * The length of time Sirius has known Pettigrew is hiding at Hogwarts is inconsistent. From Mr. Weasley's conversation at the Leaky Cauldron in chapter four, Sirius has known about it for several months. But in the Shrieking Shack in chapter eighteen, Sirius says that he first learned about it the night he escaped from Azkaban. (This is incorrect, Mr. Weasley is still assuming Sirius is after Harry, not Pettigrew, and Sirius, while sleeping, mumbles "He's at Hogwarts".) (He looks at "Scabbers" on the Weasely family's egypt picture in the Daily Prophet and instantly recognises Wormtail. Hence, he knew about it for months.)
 * On the Marauder's Map, the possessive form of "Marauders" is incorrect. Since the Marauders were a group of students (thus, the term is plural), it should be "Marauders' Map."
 * Note that the book was written before it became a commonplace term to refer to the group as the Marauders; nowhere in this book are they referred to as such.
 * Also note that the title might reflect that it is a Map for use by a marauder, as opposed to a map made by the group called Marauders.
 * 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 snuck into an office and retrieve the information of their results or were assuming that they got low marks.
 * It says "Ron crawled to the four-poster and collapsed onto it". But when Professor Lupin comes in, it says "his eyes flickered over Ron, lying on the floor". This could not be, as Ron should still be on the bed. Crookshanks was on the bed, and 'on the floor beside him.... was Ron' Ron moves from this position to 'protect' Harry. 'If you want to kill Harry, you'll have to kill us too' At this point he is standing, 'clinging painfully to Harry to stay upright'.
 * Mr. Weasley says that Ron and Harry have ended up in the Forbidden Forest twice. But only Harry has been in the forest twice. Ron went once to see Aragog with Harry in Chamber of Secrets, but the other time Harry went was for detention in Harry Potter and the Philosopher's Stone, and Ron wasn't there (he was in the movie, but not the book).
 * In chapter fifteen, during the Quidditch match, three quarters of the crowd supported Gryffindor but it's also written that there were 200 hundred people supporting Slytherin. Assuming that all eight dormitories share similar numbers to Harry's, this would be impossible, as there are not 200 hundred Slytherins or 800 students in the school.
 * Chapter sixteen (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 June 6th, 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 June 9, 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 crescent when it should be a full moon.
 * When Lupin looks at the Marauder's Map and sees Harry, Hermione, and Ron going to Hagrid's hut for Buckbeak's appeal, the Harry and Hermione from the future are not far behind them. Doesn't this mean that Lupin would have seen two sets of Harry and Hermione on the map? Also Snape saw the map and would have seen the same thing. (Professor Lupin claims the map never lies), although it could have been the time-turner's magic or possibly Lupin kept his eyes on the trio and ignored the other two names.
 * 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 movie). Although, it could be because Hermione is sitting behind him.
 * Sirius Black says that Voldemort has been in hiding for fifteen years. He's 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. Of course, Black could just be rounding off to an approximated number (fifteen years), but this seems unlikely since he keeps talking about being in Azkaban for twelve years, being that the two time frames are almost exactly the same. Sirius also says to Pettigrew 'you haven't been hiding from me for 12 years, you've been hiding from Voldemort's old supporters'. Voldemort could, however, have been hiding to escape capture and operating secretly before killing Lily and James Potter.
 * In the first book, Hagrid was quoted saying this: "There hasn't been a single witch or wizard who went bad who wasn't in Slytherin". However this does not make sense as Hagrid believed Sirius to be guilty and he was in Gryffindor. Later, we discover that he was actually framed by his old friend Peter Pettigrew. Both were Gryffindors, making this statement that only Slytherin went bad untrue. This, however, could have been just the usual prejudice expressed to Slytherins by some wizards such as Hagrid and Ron.
 * In the last chapter on page 284 there's a spelling mistake. It says Dumblefore instead of Dumbledore.
 * On the back cover it says Harry blew up his horrible Aunt Petunia instead of Aunt Marge.
 * Ernie Macmillan's last name is misspelled McMillan.
 * In Chapter 14, when Harry is trying to sneak off to Hogsmeade via the back of the statue of the one-eyed witch, he meets Neville. A while later, Snape finds them there. We are told that Neville was looking at something (Snape) over Harry's shoulder. This means that Snape was behind Harry, but soon after we are told that Neville takes a quick step behind Harry. This would mean that Neville would have just become much closer to Snape, instead of getting further away, which was obviously his intention.
 * On page 79 of the American paperback, it states: "Draco Malfoy and Harry had been enemies ever since they had met on their very first train journey to Hogwarts." It is correct that they were enemies since their first train journey to Hogwarts, but incorrect that they first met there. They first met in Madame Malkin's Robes for All Occasions when they were being fitted for robes (page 76-79 of Sorcerer's Stone, American Paperback)
 * On page 379-380 of the American paperback, 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.
 * In the American version, page 72, first paragraph, it states that "Harry and Ron led the way to the end of the train, past packed compartments, to a carriage that looked quite empty. They loaded the trunks onto it, stowed Hedwig and Crookshanks in the luggage rack, then went back outside..."

Harry Potter and the Goblet of Fire

 * Towards the end of the book when Voldemort puts the Imperius Curse on Harry the book says "For the third time in his life," when towards the middle of the book it states Harry had the Imperius Curse put on him four times by Moody (Crouch).
 * Harry's fourteenth birthday is on July 31, 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 wasn't released in Europe until September 29, 1995. It could not have been imported either, because the PlayStation wasn't released in Japan until December 3, 1994. However, it is possible that the Dursley's 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.
 * At the ending of the book Harry, Hermione and Ron travel on the "carts without a horse" and then Harry cannot see the thestrals, even though he has already seen Cedric and Lily die. The reason Rowling has given for this is that Cedric's death hasn't yet fully sunk into Harry's mind. Also, Harry did not really see his mother's death as he was in his cot at the time of her murder as it states in Harry Potter and The Deathly Hallows, where he sees into Voldemort's mind and saw the night his parents died.
 * 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.
 * In the first task, nowhere has it been stated that one could not retrieve the Golden Egg through the use of a Summoning Charm, but none of the champions even attempted this most simple system to do so. However, it is stated that the first task was designed to test the champions' daring and courage; thus, using a Summoning Charm would be too easy – almost to the point of cheating. It is possible that use of this spell would have resulted in a bottom score, or that there were enchantments on the eggs to prevent summoning. The champions may also have simply overlooked this, looking for complex solutions when none were needed.
 * 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. 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. Calculated correctly, the World Cup that the Weasleys, Hermione, and Harry attend is actually the 131st, and it should have been held in 1993.
 * Maybe the Ministry was too scared or too busy to hold the Quidditch World Cup in 1993 with Sirius Black on the loose.
 * In regards to the number of World Cups, earlier World Cups may have been held every year, with the four-year rule being only newly applied.
 * 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. Either that or Hagrid originally started off as Ogg's assistant.
 * 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. Either that or with his position in the Ministry, it is possible that he obtained a time-turner for his son. 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 is enervate. The correct spelling is ennervate, as enervate would have the opposite effect.
 * The notice that says classes will end a half hour early for the arrival of the Durmstrang and Beauxbatons students states that October 30, 1994 is a Friday. However, it was really a Sunday.
 * In the American paperback 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 Harry Potter and the Chamber of Secrets American paperback 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.
 * In the American paperback on page 461, it states: "It was Moaning Myrtle, who was usually to be heard sobbing in the S-bend of a toilet three floors below." However, in Chamber of Secrets American paperback page 230, Moaning Myrtle says, "I was just sitting in the U-bend, thinking about death..." meaning that she normally sits in the U-bend, not the S-bend.
 * In the American paperback page 498 (Ch 26 - The Second Task) it says that Ron was tied between Hermione and Cho Chang.  After Harry cuts Ron loose, it says on page 500 that Cho's head was on Hermione's shoulder, but wouldn't they be too far apart for her head to be resting on Hermione's shoulder?

Harry Potter and the Order of the Phoenix

 * 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, as J. K. Rowling has admitted that math is not her strong suit.[8] 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.
 * 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.
 * 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 snuck out of the school with his brother.
 * In chapter 9, when Harry lets Hermione borrow Hedwig, Harry remains bent double until he hears Hermione close the door. However, when he hears Hermione and Hedwig had gone, he hurries across the room and closes the door, but the door had already been closed.
 * In chapter 30 of the American hardcover, 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. They may also have just been in the topmost row of students and there may have been empty rows behind them. (Corrected in later editions)
 * When Hagrid returns and begins teaching his class again, he comments that he has been bringing Thestrals for his fifth years' O.W.L.s for years. But he only became a teacher two years ago, in Prisoner of Azkaban.
 * But he has been a gamekeeper for many years, and it is very much possible that teachers have asked him to bring them over the years.
 * In chapter 37, after Sirius' death, Harry reminisces about when Sirius watched him play Quidditch in Harry's third year. Harry wonders if Sirius had come to see if he was as good a player as James had been, and regrets never having asked him. However, in Prisoner of Azkaban Sirius specifically tells Harry that he did watch him for that exact reason, and that he was as good as his father. It is possible that Harry could have forgotten this.
 * During the Ordinary Wizarding Level examinations, Hannah Abbott was doing her transfiguration practical at the same time as Harry Potter. This is odd as the examinations are done by surname, meaning that Hannah should have finished a while ago, unless she was a rather incompetent student.

Harry Potter and the Half-Blood Prince

 * On page 225, it states that Jimmy Peakes and Ritchie Coote join Katie, Demelza and Ginny in the stands for the selection of their last team member, the Keeper. Yet, on the next page, when McLaggen missed the 5th goal, he told Harry concerning Ron's success, "His sister didn't really try.... She gave him an easy save."
 * On page one-hundred and one of the British version 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, as she dropped both Divination and Muggle Studies in her third year, she couldn't possibly have got eleven O.W.L.s as 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.
 * On Harry's sixteenth birthday, Remus Lupin tells Harry that Ollivander has gone missing. But in Harry Potter and the Deathly Hallows, it said that Ollivander had been in the Malfoys' cellar "a year". However, if he had been captured at the beginning of the sixth book, he would have been held in the cellar for about two years. However, this could mean that Ollivander was held in a separate place for a year before being moved to Malfoy Manor.
 * Harry's sixteenth birthday happens on July 31st, 1996. Looking at any full moon calendar, in that year full moon was on July 30th. 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. December 24, 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 doses of Wolfsbane potion required to make him harmless. Therefore, on that night he would have been a dangerous transformed werewolf.
 * In the chapter titled "The Secret Riddle", ". . . 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." Of course, since Dumbledore is a wizard, this may not be a mistake at all.
 * Hermione (approximately as tall as Harry) reaches out to pat Hagrid's shoulder, later Harry can only reach as high as his elbow. (It is possible that at the time Hermione patted Hagrid's shoulder Hagrid was seated and when Harry patted Hagrid's elbow, Hagrid was standing.)
 * On page 342 of the American version of Harry Potter and the Half-Blood Prince, 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 is corrected in the newer editions, however.
 * When talking to Katie Bell about Quidditch, Harry says to her, "you don't need to try out, I've watched you play for five years...", to which Katie replies, "You mustn't start off like that, for all you know, there's someone much better than me out there. Good teams have been ruined before now because Captains just kept playing the old faces, or letting in their friends..." However, there were never any tryouts mentioned in Harry's second or third year, and the same team members played for all three years. In fact, the first tryouts mentioned are in 1995 when they need to get a new Keeper, and this was the only position that changed at the time. In addition, Harry had only watched her play for four years, since Quidditch was cancelled in his fourth year for the Triwizard Tournament.
 * When Harry asked Lupin if the Half-Blood Prince was James or Sirius, Lupin suggested that Harry see how old the book was. Harry discovered that it was published fifty years earlier, and "neither his father nor Sirius had been at Hogwarts fifty years ago." Later, Harry found that Severus Snape was the Half-Blood Prince. However, Snape went to Hogwarts at the same time that James and Sirius did, not fifty years earlier.
 * However, it is possible that Snape either purchased a used copy of the book, or may have been given his mother's copy from when she was a student at Hogwarts. It is known that the Snape family was not well off.
 * At the beginning of Dumbledore's funeral, Harry says he's never been to a funeral before. However, in Goblet of Fire , he sort of went to a funeral at Cedric's Memorial Feast, and in the film , it practically was a funeral.
 * When Harry and Ron are speaking with Fred and George at the Burrow, George says they're off to see a "pretty [muggle] girl who thinks my card tricks are something marvellous... almost like real magic...". However, if George did indeed use magic he would have been breaking the Statute of Secrecy.

Harry Potter and the Deathly Hallows
This is a mistaken error. The Fidelius Charm was placed on Shell Cottage and Auntie Muriel's after Harry, Ron and Hermione escaped Malfoy Manner.
 * 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.
 * When Harry is viewing Snape's memories, he witnesses a scene on Platform 9 3/4 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.
 * When Harry stuns Umbridge and Yaxley in the courtroom, he "whirls around" to see dementors approaching Mrs. Cattermole. But Harry was already facing her, having been seated behind Umbridge, Yaxley, and Hermione.
 * When Harry talks to Griphook about breaking into Gringotts, Griphook has the Sword of Gryffindor in his hands. When Harry stands up to leave a few moments later, the sword is now beside Griphook's bed.
 * At the beginning of chapter two, it says that Harry can't use magic due to not coming of age for 4 days. In chapter six, days have passed since the Battle over Little Whinging, but Harry states the Trace will break in 4 days.
 * When Harry, Ron, and Hermione leave the Great Hall after the Battle of Hogwarts ends, Harry is under his invisibility cloak, and remains under it when they reach the Headmaster's office. When Harry raises his arms to silence the headmaster and headmistress portraits, he is suddenly visible to everyone, but there's never any mention of him taking the cloak off.
 * When Hermione erases the Death Eaters' memories in the cafe she states she has never erased a memory before. However, she erases her parents' memories before going to the burrow. (This should not be a mistake because J. K. Rowling had said that she had used another spell. She had taken away her parents memories. They could get them back if she had reversed the spell. With 'Obliviate', you might not be able to).
 * 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 later after the Death Eaters arrive Hermione says "Harry give me the Invisibility cloak, Ron you're going to put it on." However it is possible that Harry was given the cloak again without a mention about it in the book (for example he could have taken it while Hermione was reading The Tale of the Three Brothers)
 * Also, in the Room of Requirement when Harry, Ron, and Hermione are talking with Malfoy, Crabbe, and Goyle, it says that Crabbe's voice was soft. But in the second book, when Ron turns into Crabbe after drinking Polyjuice Potion, Ron's voice also changes along with the appearance. So his voice became Crabbe's, which was said to be like a grunt. So how could Crabbe's voice be a grunt when he was twelve but then soft when he was seventeen or eighteen, unless of course he went under an abnormal voice change where his voice goes from lower to higher instead of higher to lower. It also says that Harry had hardly ever heard Crabbe speak before, but he would have heard his voice a lot when Ron spoke after taking the Polyjuice Potion in their second year at Hogwarts.
 * In chapter twenty-three, Narcissa says that Draco "is home for his Easter holidays." In chapter twenty-four, 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." Since Easter never comes until the Vernal Equinox, and Hogsmeade can hardly be more than a bit west of Shell Cottage, the sun should have risen at Hogsmeade by the time it rose at Shell Cottage. And even if the sunrise was blocked by a mountain, the sky would be light.
 * When Ron returns after having walked out on the other two, he informs them about the Taboo that has been placed on Voldemort's name, and that saying his name would break any protective enchantment placed on and around the person, and that this was how the Death Eaters had found them so easily on Tottenham road. This does hold true, when the trio are later captured by a gang of snather's. However, this Taboo fails to come into effect when they take Voldemort's name at Grimmauld Place, as the Fidelus Charm continues to hold.
 * When Dobby arrives in the cellar at Malfoy Manor, and Harry wonders what would be a safe place to send Dean, Luna and Ollivander to, Ron suggests Shell Cottage, Bill and Fleur's new place. Bill later tells Harry that their house has been placed under the Fidelus charm and Bill is the Secret-keeper. Since, according to the laws of the Fidelus charm, no one but the Secret-Keeper himself can give the location of the house placed under the charm, to another person, Ron shouldn't have been able to say the name of the place at all.
 * In Snape's memory, it is shown that Snape comes up to Gryffindor tower and threatens to sleep near the Potrait hole of the Fat Lady when Lily refuses to talk to him after him having called her "Mudblood". However, students are not supposed to know the locations of the common rooms of the other Houses, so it is unexplained how Snape could go up to the Gryffindor common room. However, it may be possible that Lily might have told him the location, as they were best friends.
 * In a school like Hogwarts, the locations of the house common rooms could hardly be kept secret. How to access a house's common room is a secret trusted to each student in that particular house.
 * In Snape's memory, he and Lily were talking in the corridor outside the Gryffindor common room, next to the portrait of the Fat Lady, which is where Snape was threatening to sleep if he had to.
 * JK has claimed that Dumbledore died at around age 150. How is it possible that Muriel Weasley - aged 107 at the Wedding was around during the Dumbledore's childhood? Also if we go with the assumption the books are set in the 1990's there is an error with Dumbledore's age. He claimed he never saw Grindlewald for 5 years after the death of Ariana which occured when he was about 19. The dual took place in 1945. Therefore he must have been born in the 1920's so cannot be 150!
 * Harry and Ron never passed their apparition tests, but yet they apparate.
 * However, as they are able to perform the magic prior to passing the test, they are simply apparating illegally with no licenses. This is hardly an issue, as they are wanted fugitives anyway.
 * The pensieve is used to store memories and view them. All of Snape's memories are extensive and explain the whole story to Harry, but the only part of that which Snape hid from Harry in Order of the Phoenix was the part about Harry's father embarrassing Snape. Further, it's unclear whether these memories are removed or just copies. Dumbledore gave his memory of the prophesy to the Ministry, but still had the original memory. Same for Slughorn who gave Dumbledore a modified Horcrux memory, but still had the original. Therefore Snape could not possibly hide all his knowledge from Harry if something went wrong (and did) during Occlumency lessons.
 * Harry did not have access to every single one of Snape's memories at will; it's likely that Harry simply stumbled upon that particular memory at random when he accidentally entered Snape's mind.
 * It is clear that storing memories in the pensieve does not remove the original memory in your mind, otherwise there would be no point to using a pensieve in the first place. The memories that Dumbledore, Snape, and Slughorn extract are just copies.