Surely Quirrell should have considered the possibility that Harry would come up with a surprisingly powerful move. It's at least plausible that Quirrell's plan was too brittle.
Actually, judging from his rant upon being awoken, his error was that he overestimated Harry. He thinks that someone as smart as Harry should have realized it would make no sense to kill the auror.
Essentially Q's error was simply the Usual Error, i.e., assuming that others think in a way similar to ourselves, and especially, that they will find our intentions or conclusions equally obvious. To Q, it was obvious that nobody would be dumb enough to kill the auror under those circumstances.
That's assuming Quirrel is telling the truth, though. If he didn't intend to kill him, why use the Killing Curse? If the goal really was to subdue, to dominate, this doesn't seem to be the logical approach: The battle was almost won at that point, surely using other attacking spells would have been almost as successful. I think real-life fights could be used as analogy: If you intend to subdue, not kill, in real life, you use a taser/pepper spray/whatever, not a gun.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.