A lesson in code smell and good style: I took one look at the code and (before noticing the bug) thought, “why is there an inner if instead of just using a single conditional with a logical AND?”

That is to say, I could have fixed the bug without knowing what it was. (I'm a hs student and don't do much bug-shooting in code that wasn't written by me or my close friends, so I don't know if this is common.)