Plot hole: On page 56 of the original UK edition, Harry overhears a witch complaining about the price of something as being "seventeen Sickles." There are seventeen Sickles to a Galleon, so a witch who is familiar with the Wizarding world (as she obviously is, given that she knows that price to be unusually expensive) would say "one Galleon" instead. This was changed to "sixteen Sickles" in later editions for this reason. Page 72 in the American version.
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.
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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.