Continuity mistake: Harry buys the book "One Thousand Magical Herbs and Fungi", but later, the title changes to "One Hundred Magical Herbs and Fungi".
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.
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.
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.
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.
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.
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.
Tailkinker ★