Way way way way back in chapter 3, after McGonagall has told Harry about Voldemort's attack on him and his parents, it says:
(And somewhere in the back of his mind was a small, small note of confusion, a sense of something wrong about that story; and it should have been a part of Harry's art to notice that tiny note, but he was distracted. For it is a sad rule that whenever you are most in need of your art as a rationalist, that is when you are most likely to forget it.)
Is the "something wrong" simply that if the story is correct then there were no surviving witnesses to report it (it's far from clear to me whether this should be a fatal objection, given the existence of magic; still less clear, presumably, to Harry), or is there meant to be something else obviously-if-you're-thinking-clearly wrong about it?
There are loads of fan-theories as to how they would know specifically; my favorite is, based on how confidently Dumbledore explains exactly what happened that night, that he used Legilimency on the newly orphaned Harry. Another fairly prevalent one is that the scar Harry has is unique for being the result of a Killing Curse, yes, but curse scars in general are not unique at all. They also could have used any number of detection spells--honestly, there are a multitude of options.
What I thought was wrong was that McGonagall explicitly said that the Killing...
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
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. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
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: