Other mistake: On page 221 of the American hardcover, the third paragraph begins "...wished him luck outside the locker rooms the next afternoon." But Harry couldn't have just been getting to the locker room in the afternoon; Quidditch matches take place in the late morning: the school teams are usually dressed and already on the pitch by 11:00 am.
Continuity mistake: Harry buys the book "One Thousand Magical Herbs and Fungi", but later, the title changes to "One Hundred Magical Herbs and Fungi".
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.