MISTAKES
During the sorting in book 1 the first years sit on a three legged stool but then in the third book they sit in a four legged stool.
In Book 1, when Harry is playing Quidditch against Slytherin, they mention that the captain of the Slytherin team is Marcus Flint, a sixth year. In the third book when they play against Slytherin in the finals, Flint is still the captain of the Slytherin team. This would make him an eighth year and he should no longer be at Hogwarts.
In Book 1 Hagrid stated that he was going to take Sirius Black his motorcycle back, and then leaves. However, in Book 3 Hagrid says that Sirius Black gave him that motorcycle to get Harry to Vernon and Petunia. Hagrid says Sirius told him he would not need it any more.
There was no full moon on June 4, 1994 in fact, there was barely any sign of a moon at all (that is the day Lupin turns to a wolf) but there was one in 1993. [Brian writes: Harry would have been in his fourth year in 1994. His birthday is in 1980, making his third year 1993, where there was a full moon.]
During the Quidditch Final Katie had put another penalty past the Slytherin SEEKER. But the position that blocks the goal is the KEEPER.
In Book 3 it says that they haven't studied werewolfs yet. But in book 1, chapter 13 they are coyping down "different ways of treating werewolf bites".
When Sirius (the dog) was pulling Ron into the Whomping Willow and Harry and Hermione followed them, Lupin was reading the Marauder's Map. He should have seen Harry and Hermione on the map at 2 places because they were there using the time turner to save Sirius. Why did Lupin AND Snape not notice that?
In book 3 on the Hogwarts express when Malfoy walks into the room it says "Draco Malfoy and Harry had been enimies eversince they had met on their very first train ride to Hogwarts." but in Book 1 they met in the robe shop. [Brian writes: Harry wasn't enemies with Draco when they were in the robe shop. It wasn't until after that on the train ride that they became true enemies.]
There are two different versions of how they came out of the Wooping Willow with the time-turner. The first time it says that It went Lupin, Pettigrew, and Ron, then Snape, then Harry and Hermione, then Black. But in the second version when they were watching themselves it says it went Lupin, Ron , and Pettigrew
together, then Hermione, then Snape, then Harry and Black.
New! Evemarie: In "Talons and Tea Leaves" Harry tells Hermione and Ron that he saw the grim on Privet Drive. Although, in "The Marauder's Map" it says that Harry didn't tell anyone about the grim.
New! Ila: On page 370 of the US edition, Sirius Black is talking to Peter Pettigrew and says, "Voldemort's been in hiding for fifteen years, they say he's half dead. You weren't about to commit murder right under Albus Dumbledore's nose, for a wreck of a wizard who'd lost all of his power, were you?" This bit about 15 years doesn't make sense. Harry's supposed to be 13 in this book. Voldemort killed his parents when he was a baby, so Voldemort should have been in hiding for twelve years and about nine months (Didn't Voldemort kill them on October 31?). So unless Voldemort was in hiding for years before Harry was even born (which is a possibility, maybe I'm not reading the books right), the 15 years doesn't make sense.