When H&C drops the disguise, Hermoine recognizes him/her. I don't think it is particularly likely that ghost-Voldemort looks anything like any picture of him from a history book. So how would Hermoine recognize him?
I feel that I should point out that when the black mist lifts and Hermione recognizes the face of her assailant we have no reason to believe that the face she recognizes is not itself an illusion.
Since we already know that she has been obsessing about Draco, I suspect that it may even have been his face (though with the information we readers have it is obvious that H&C is not actually he), though I don't put a great weight on that suspicion.
(The HPMOR discussion thread after this one is here.)
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. There haven't been any chapters recently, but it looks like there are a bunch in the pipeline and the old thread is nearing 700 comments. The latest chapter as of 7th March 2012 is Ch. 77.
There is now 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.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
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: