MISTAKES
It was mentioned in the books that when victims appear from wands they come out the reverse order they died. And everyone should know that James Potter died before Lily Potter. But when that happened with Harry's and Voldemort's wand, James came out first. Lily was supposed to come out first. [Note: this mistake was corrected in later printings of the book]
On page 685 (US) Alastor Moody has just been revealed as Barty Crouch in disguise. Then they find Moody in the trunk, stunned, and they don't wake him up. But on page 685 it says, ' The Imperium Curse, said Moody' but it was really Crouch who was talking.
Just before the last task, the weasleys come to visit Harry and he gives them a tour around Hogwarts. it says that Mrs Weasley remincied about the gameskeeper called Ogg, when in fact, the gameskeeper in her school years should've been Hagrid, unless Mrs Weasley is more then 80 years old!
When Harry and Cedric have travelled from Hogwharts to the graveyard with the Portkey and Harry falls over he can hear the high pitched voice say: "Kill the spare". That indicates that Wormtail killed Cedric. Yet Cedric appears from Voldemorts wand later.
On page 229 it says that Miss Fawcett is in Ravenclaw, but Snape takes 10 points from Hufflepuff because of her shenanigans.
On page 400 it says that Myrtle cries in the "S" Bend of the toilet, but in book 2 it says she cries in the "U" bend.
When Harry is practicing for one of the tasks, Hermione says something about practicing some simple hexes over the summer... According to laws made by the Ministry of Magic and Hogwarts School, students are not allowed to use magic over the summer.
Mad-Eyed Moody says that Lily and James Potter were killed by "Avada Kedavra". When others were killed using that curse, they fell to the ground and were just dead. In Book 1, Aunt Petunia remarks that her sister got herself "BLOWN UP" and Hagrid said in the same chapter that the house was in ruins. If Lily was killed with Avada Kedavra she would have just died.
Bagman says he is setting up Portkeys over 5 continents. there are 7 continents but only 6 are inhabitable. Do you know which continent misses out? They talked about European withcs/wizards, African ones and North American ones. What about Australia, Asia and South America?
Harry uses lemon drop as the password in "The Madness of Mr. Crouch" and fails to get in. Then he uses it again in The Dream. If it didn't work, why'd he use it again?
On p. 712 (US Edition), Dumbledore is explaining to Sirius and Snape the need for them to call a truce and work together: "...You will shake hands. You are on the same side now. Time is short, and unless the few of us who know the truth do not stand united, there is no hope for any of us." What that means, technically, is that as long as they stand together then there is NO hope. But Dumbledore wants to say the opposite, yes? Therefore, it should read, "unless the few of us who know the truth stand united, there is no hope for any of us."
Hermione says she gets the Daily Prophet again because she doesn't want to hear the news about herself from the Slytherins, but now I remember in book III, in the second or third chapter Hermione also gets the Daily Prophet!
At the end of Book 3 Ron recieved an owl, for keeps as a present, from Sirius Black and Harry and Hermione were both there to see it. Then near the beginning of the 4th book Ron sends Harry a letter with the bird and Harry seems as if he had never seen it before.
Karkaroff stated that Moody had once smashed one of his own birthday presents to pieces, believing it to be a basilisk egg. However, in book 2 on the library book page in Hermione's hand, it says that a basilisk is "born from a chicken's egg, hatched beneath a toad". If a basilisk is born from a chicken egg, then there is no such thing as a basilisk egg.
There is a sentence that says that Lord Voldemort was the most feared Dark Lord for a century. Lord Voldemort was sixteen years old 50 years ago (which we know because of book 2), when he was at school, you add that together and you get 66 years. How can he be the most feared Dark Lord, for longer than he has been alive? [Note: I think it would be more accurate to say "there hadn't been a more feared Dark Lord in a century."]
When Bartemius Crouch drinks the Veritaserum in the end he tells that after he got the map from Harry he waited for a week before his father arrived. But He got the map right after Harry solved the clue with the second task. That means that he got it before February the 24th. And Mr Crouch arrived to Hogwharts right after the champions found out about the third task. And that wasn't until May the 24th. So it's at least two months.
When Harry and Voldemort's wand do "priori incantatem", the first spell Voldemort's wand "regurgitates" is "a dense smoky hand flew out of the tip of it and vanished... the ghost of the hand of wormtail..." Well Voldemort didn't cut off Wormtail's hand. He did it himself with the dagger to make the potion! [The shadow of Wormtail's hand isn't from when Wormtail cut it off, it's from when Voldemort replaced it with a new silver hand.]
The clock at the Weasley's has been described in two different ways in book 2 and book 4. In the second book, its described as having one hand, with things like time for dinner, time for sleep and so on. in the the fourth book, i.e. Goblet of Fire, its described as having a hand each person in the house, with things like mortal peril and so on, where the numbers ought to be with each hand moving according to whatever activity they were doing at that time.
In the second chapter, Harry awakens early in the morning from a vivid dream about Voldemort, Wormtail & Frank the gardener, and his scar is hurting him badly. Later that morning a letter turns up inviting him to go to the Burrow for the rest of the hols, and to attend the World Cup. After breakfast Pigwidgeon turns up with a note from Ron, telling Harry they will be picking him up on Sunday at 5.00pm. Harry replies and says "see you tomorrow". This means his Scar hurt him on the Saturday. However, chapter Ten, Mayhem at the Ministry, page 133, sees Harry telling Ron & Hermione for the first time about his Scar, but he tells them his Scar hurt him on SUNDAY. I guess Harry isn't very good with days of the week!
When Harry, Ron, and Hermione went into the forest at the Quidditch World Cup, Hermione cast the spell "lumos" so they could see. Students at Hogwarts aren't allowed to use magic over the summer.