Corrected entry: Chapter Nine: Hermione says that Snape mentioned Polyjuice Potion "in class a few weeks ago." He said that the potion is described in the "Moste Potente Potions" book. This is pure plot convenience. There is no practical reason for Snape to tell second-year students about a risky, complicated potion, nor mention a book that contains the recipe. He is cruel and unfair, but not blatantly irresponsible.
Corrected entry: The Hogwarts' school supply list mentions the Lockhart book, "Wanderings With Werewolves," but later, Lockhart refers to the book as, "Weekend With a Werewolf." (May be corrected in later editions of the book).
Correction: Lockhart wrote many books and is likely referring to another one about his (fake) experiences with werewolves. The two titles are quite different from one another and a weekend with a werewolf is not the same as "wandering" with them, which would happen over a longer period of time. It therefore seems that these are two different titles.
Corrected entry: Ron and Harry both fly the Ford Car straight into Hogwarts Grounds. However, Dumbledore has strict spells bound onto the Castle border that prevent anything from flying over the castle walls.
Correction: He cast those protective spells in Book 3 (due to the escape of Sirius Black). This is the second book.
Corrected entry: Ron said that Fred turned his teddy into a spider when he was 3 years old. But Fred would have been 6 or 7 at the time and wizards don't get their wands until they are 11.
Correction: In the very first book, Harry was able to do magic without a wand, albeit unintentionally. Why is it so difficult to believe that the son of a major wizarding family might be able to do the same?
Correction: He could have taken one of his parents' wands.
Adding on to this, Ron uses Charlie's old wand when he starts school since his parents can't afford a new one. So, as you point out, Fred could have easily taken one of his parents' wands.
Corrected entry: Hermione says the lacewings for the Polyjuice Potion have to be stewed for 21 days. But on page 184 Ron pours the dead lacewings on top of the leeches. They couldn't already add the lacewings, as it is only the first day.
Correction: It doesn't say whether the lacewigs have to be stewed seperate from the potion or added to the potion and stewed there. Since he adds them on the first day, we can assume the latter.
Corrected entry: The picture at the beginning of "The Rogue Bludger" shows Harry's left arm deboned. However, it says his right arm was the one that got deboned.
Correction: Artwork is not part of the text, and should not be counted as book mistakes. The same goes for artwork on videos and DVD's on moviemistakes.com, they are not considered parts of the movie and are not accepted as mistakes.
Corrected entry: When Harry enters Tom Riddle's memory of catching the "Heir of Slytherin," he is in the headmaster's chamber. but Tom isn't in the room yet. He enters shortly, but how can Harry see Dippet if Tom wasn't there to remember it?
Corrected entry: In this book Percy takes 10 points from Gryffindor, when Ron Harry and Hermione are outside the girls bathroom on the second floor. "Ten points from Gryffindor" said Percy, fingering his prefect badge. However, later in the series, once Umbridge begins teaching, Malfoy tries to dock points from Harry, Hermione and Ron, and Hermione says "Prefects can't take points from students, Malfoy, only teachers can".
Corrected entry: Hermione has mentioned several times (but this may be in later books) that nobody can Apparate or Disapparate to/from the Hogwarts grounds, but Dobby frequently does this. It may be that the rule only applies to humans, but surely a clever wizard like Dumbledore would make sure that the Apparating/Disapparating rule applies to all creatures and not just humans.
Correction: This has already been submitted and corrected several times. J.K. Rowling herself has stated that House Elfs use a different kind of magic, so they can appear and disappear in and out of Hogwarts.
Corrected entry: During the Flourish and Blotts scene, Harry is presented with a complete set of his needed books by Lockhart. He then dumps these into Ginny's cauldron, mumbling 'I'll buy my own'. But before he is able to, the fight breaks out and they then immediately leave the store, travelling home by Floo powder without re-entering the store. Where did Harry get his books from, then?
Correction: In the "Half Blood Prince", we learn that students are allowed to order books after they are already at school. Harry and Ron have to get their potions books. It makes sense to assume that Harry or someone else at the school was able to order the books for him.
Corrected entry: On page 182 (page may be different for different versions), Harry is still in the hospital, yet when he wakes up, it says the dormitory was blazing with winter sunlight.
Correction: It's the hospital dormitory. It can't just be classed as a room, because there are several beds, due to the large number of injuries that happen...
Corrected entry: On page 105, Harry finds Mrs Norris petrified on the second floor. When Dumbledore asks them all to follow him, Lockhart tells Dumbledore to use his office, and he says that it's just upstairs, which would mean Lockhart's office is on the third floor. But on pages 91-92 it says that Harry is dragging his feet along the second-floor corridor to Lockhart's office for his detention.
Correction: Just because he was 'dragging his feet along the second floor corridor' does not mean Lockhart's office is there. he could have gone up a flight of stairs to reach Lockhart's office.
Corrected entry: It states in the book that ghosts can't eat or drink, so how is the Mandrake Potion administered to Nearly Headless Nick?
Correction: I'm sure the great wizards at Hogwarts could find a magical way. Some items can be left to the imagination.
Corrected entry: In the chapter 'The Rogue Bludger', Harry breaks his arm in the quidditch match against Slytherin and Madam Pomfrey said "This boy needs rest, he's got thirty-three bones to regrow!" There are only 30 bones in an arm.
Correction: There are 30 bones in the arm and hand. Add in the clavicle and scapula for two more. There are also likely to be sesamoid bones - bones embedded in tendons or muscles that develop in response to stress or strain. Harry has certainly been through plenty of both. She could also be referring to the epiphyseal plate, AKA the growth plate.
Corrected entry: 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.
Correction: There is no reference to Harry being on the second floor when he sees the writing.
But just before he finds Mrs Norris and sees the writing, it says they sprinted up the stairs to the second floor.
Correction: Why would Snape think any second-year student would actually be able to make the potion? Not irresponsible at all for him to mention an advanced potion like that to second-year students. Mentioning it doesn't seem all that unlikely either; he is, after all, the potions teacher. Maybe he just listed a few complicated potions as examples compared to easier potions the second years are taught to make.
lionhead
I would like to add that Snape believes a "select few" students will appreciate the art of potions. He may have mentioned this potion and book, in case it catches the attention of a "worthy" student.