It's not much of a mistake, but in the American Large Print Edition of Harry Potter and the Prisoner of Azkaban, Chapter Fourteen, "Snape's Grudge", in the sentence "Snape picked up the Zonko's bag", it instead says "Snap picked up the Zonko's bag". Whoops!

From J.K. Rowling's commentary on how Mrs.Weasley is not a "mom", but a "mum" (and all the other mothers mentioned in the books), I know that even though I have the American edition of Harry Potter Year 1, all the "moms" should be "mums", but they aren't. I guess I have an older copy (later on the editors fixed all the "moms").

Ron mentions Ernie McMillan (Prisoner of Azkaban,LP Edition) but there is a Hufflepuff in Book Two and Book Four called Ernie MacMillan.

In Book Four, Voldemort, as a snake, 'holds' a wand to perform the "Avada Kedavra" curse... but snakes don't have hands or paws! Sure, they could wrap their end around the wand to perform it, but it just says hold...

Anonymous: We know that Harry's birthday is on July 31 because that is the same day as the break in at Gringotts. From the second book we know Nearly Headless Nick's 500th Death Day Party was on 10/31/1992. Doing the math, we know that Harry's 11th Birthday was 07/31/1991. In The Sorcerer's Stone, Harry says his birthday is on a Tuesday, the day after Dudley's favorite TV show (The Great Humberto). The mistake is that July 31, 1991 fell on a Wednesday, not a Tuesday like the book said!

Anonymous: In the beginning of the 4th book it states Dudley threw his Playstation out his window. Since Harry was born in 1980, the year would be 1994, but Sony Playstation wasn't released yet.

If Moaning Myrtle has haunted the girls' bathroom (well, the U-bend of a toilet) for fifty years (since she died), then why wasn't she mentioned in the attack of the troll in Book One which takes place in the girls' bathroom?
Correction: In Goblet of Fire, Myrtle mentions to Harry that she had been haunting a Hogwarts schoolgirl in Tom Riddle's time bullied Myrtle and then found her dead. Eventually, Myrtle returned to her toilet, but probably didn't until that year.

In the back of my paperback copy of Chamber of Secrets, there's an ad for Book 3, Prisoner of Azkaban. The first sentence of the summary says: "Harry Potter has to sneak back to Hogwarts after accidentally inflating his horrible Aunt Petunia". Er--- it was Aunt Marge who he inflated!

In Prisoner, it reads (page 428) "It took Harry a moment to realize what Dumblefore had said." Oops!
thanks to Kevin