Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, 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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
The worry is that in the HPMR universe a lot of things which are fictional in our universe have turned out to exist. For example, McGonagall's references to prior Defense Against the Dark Arts teachers included Professor Summers and Professor Blake. In some cases the situation looks even stranger since Harry has played Dungeons and Dragons (we found that out in the visit to Diagon Alley) but Draco has heard about creatures called mindflayers which are from D&D. Making things worse, we also know that Harold Shea is a real world wizard in Harry's universe and his entire MO is to go into fictional universes from "our" universe.
The concern therefore is that there might be a creature or terrible thing that Harry thinks is fictional but actually exists. Thus it might well be that the Chaos Gods or Cthulhu or Yog-Sothoth actually exists. If so, Harry using them in a chant could have some very bad consequences.
Ah, much clearer.