I'm not quite sure it's the idiot ball, for two reasons.
1:
I would call it senility, if so many others had not missed it as well.
Even with the knowledge that Quirrel was possessed by Voldemort in canon, whether or not Quirrel was Voldemort was an active topic of debate among readers for quite some time.
2:
Really, am I that hard to recognize without the glowing red eyes? ... Quirinus Quirrell seemed - what is the term I am looking for? Ah yes, that is the word. He seemed sane.
If Dumbledore is used to thinking of Voldemort as a cartoon villain, and expects his shade to also behave like a cartoon villain, I could see him missing Quirrell.
(There is, perhaps, also the third reason of motivated cognition brought on by McGonagall, but I always saw that as more plausible if it were magically brought on by Riddle than her really not caring who he was.)
There's also the possibility brought up earlier and echoed in a lot of the other threads: this may be a Dumbledore spawned by the mirror as part of reading Quirrell's mind and determining his CEV. That doesn't explain how real Dumbledore didn't see it, though.
That doesn't explain how real Dumbledore didn't see it, though.
Dumbledore may have seen it, and refused to let on. Quirrell is next to impossible to kill permanently. He says he can detach from a body if it is in danger, and if you kill the body, he detaches too.
The argument would go that Dumbledore was luring Quirrell into the Mirror as a final solution, and therefore pretended not to recognize him.
(Though wouldn't an easier play be a sneak attack by Dumbledore incapacitating Quirrel but not killing him? )
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 110.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: