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.
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 ★