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: 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: Has it ever been explained what happens to a wizard/witch if they don't repay a life debt? Harry should owe one to Snape after Snape rescues him from Quirrell's curse during the Quidditch game, but he never does throughout the series.
Answer: It does not appear that life debts automatically form whenever somebody saves somebody else - J K Rowling has, for example, stated that Ginny did not incur a life debt to Harry when he saved her in the Chamber of Secrets, although she said nothing about what circumstances need to occur for a debt to exist. In Snape's case, there would seem to be several possibilities. 1) A life debt simply didn't form. 2) Snape is protecting Harry because of his love for Lily and his failure to save her, so he may actually be paying off a debt of sorts himself by doing so. 3) Harry's father James saved Snape from almost certain death when Sirius tried to trick him into going into the Shrieking Shack when Remus Lupin was in his wolf form. As such, Sirius may have owed James a debt, which he paid off by saving Harry. 4) A life debt did form but, as Snape died before Harry could pay it off and, insofar as we know, had no relatives that the debt could pass to, Harry was let off the hook.