A Patronus (incorporeal!) intercepting an Avada Kedavra spell is bizarre and ridiculous. Anything less ridiculous would not have been able to interfere. With hindsight, sure, he should have known, but he doesn't have hindsight, we do. Before the event, he couldn't have possibly planned for Harry interfering like this, and we can see he already planned for any more probable level of interference - making Harry lie down on the steps far away, ordering him not to get involved, etc. If you really honestly think he should have expected a smart eleven-year-old boy possessed of a stronger-than-usual Patronus charm to be able to deflect the undeflectable curse, then he also should have planned for some equally bizarre event such as Dumbledore breaking the Apparation wards on Azkaban in order to teleport in front of the Avada Kedavra spell, and just taking it on chin because he's invincible.
Honestly. Quirrell would have to be holding either the Idiot Ball or Batman's belt in order to prepare for this.
You're right- here's the Idiot Ball, in my pocket all along.
- 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.