According to a post on /r/HPMOR:
[EY] posted a comment a while ago that the short sad ending would have been Harry blowing himself up Transfiguring antimatter.
ETA: Found EY's relevant comment:
In the profoundly improbable event that I'd needed to write [a sad ending], it would have just been Harry suiciding via antimatter (that went off prematurely as soon as it started to Transfigure) and Hermione waking up among the flaming ruins.
That's the "sad" ending?
The Disposable Male sacrificing himself for the Greater Good and wept over by the Fair Damsel is the classic "bittersweet", ennobling ending.
The Sad Ending is Voldemort taking over Harry's body and destined to rule Magical Britain, and the world. Or Harry being compelled by his Vow to twist the Universe to some Cthuluic purpose.
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 117.
Plans for next chapter release:
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.)