Probably nothing, but it would require a series of improbable coincidences (every atom in future!Harry needs to be in the same position that past!Harry saw it in), significantly complicating the loop. Such complications would make it simpler for the loop to not happen at all, and so it probably wouldn't. A precommitment to interact with your future self as little as possible would then maximize the probability that the loop occurs in the first place.
Probably nothing
So what do you think McGonagall meant by disconcerting?
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 122, which is the final chapter of the story.
Happy once-in-a-century Pi Day! (3/14/15 == 3.1415)
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.)