Let's speculate.
Say, Harry Potter tried, failed, and sent a note to the past. What happens to the Harry in the future? He presumably continues to exist, in an alternate universe where he didn't get a note and went on with the plan.
Thus, we have a scenario where, if the test was planned for, Harry must have both Gone on the mission and Not Gone on the mission, and we're merely following the one that did in the narrative.
Do not mess with time note - This also should result in aborting the mission since some terrible paradox occurred and you DO NOT mess with time.
Such loops are stable even without any paradox. The extent to which one can deduce whether bad things happen depends on the psychology of the individual and existing priors.
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: