Someone talked about the chapter being in "fantasy mode". My first thought was that they were saying that the chapter might be a dream. This doesn't seem to be what they meant, but the chapter is so odd in terms of the overall story that I don't want to entirely exclude the hypothesis.
At that point, the hard thing is thinking of a way that it being a dream or other sort of hallucination wouldn't be completely infuriating. I'm not sure if it "Harry has an enemy who's attacking him in his dreams" would be good enough, though that would be a very Harryish nightmare. Maybe it's his subconscious telling him that Quirrell isn't entirely trustworthy.
I usually do not like dream episodes if they are not well hinted at. It often feels like a cheap excuse to blow everything up without hurting the status Q.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.