Factual error: On page 25, it mentions that the boa constrictor at the zoo winked at Harry. Snakes don't have eyelids at all, so it's impossible for them to wink.
Factual error: When Harry is with the Dursleys in the run-down shack, he realises that Tuesday is his birthday. However, in 1991, July 31st was a Wednesday.
Continuity mistake: While being sorted into houses, Harry looks up at the Sorting Stool, and there are only three people left to be sorted. Professor McGonagall then calls out the names of four more kids.
Suggested correction: This is incorrect. After Harry is selected for Gryffindor, McGonagall calls Lisa Turpin, Ron Weasley, and Blaise Zabini, three students.
This is actually a difference between the British and American releases. In the British (Philosopher's Stone), Lisa, Ron, and Blaise are called, making three students. But in the American (Sorcerer's Stone), Dean Thomas is also called, making four students; however, the book still states only three students remain.
Interesting fact; I didn't know that. I guess not calling Dean Thomas in the British release could count as another mistake.
Question: Is it true that J.K. Rowling originally intended for Hermione to have a muggle sister?
Answer: Yes. J.K. Rowling was going to give Hermione a younger sister who was a muggle. However, by the time "Harry Potter and the Order of the Phoenix" came out, she felt that it was too late.