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. Another possibility is that it was Moody that was originally supposed to be killed, and the magical trunk presented an unexpected opportunity.
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.
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). 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 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 was not released in Europe until September 29, 1995. It could not have been imported either, because the PlayStation was not 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. 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 student's 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.
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, as he was still very young when he was expelled from Hogwarts, as Hagrid himself states 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. 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 from unconsciousness is ennervate. The correct spelling is rennervate, as 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 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.
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. As 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.
