I have a theory about that, actually. If the experiment had gone just as Harry expected, would he have been able to avoid the temptation to write something different on the paper, just to see what happens?
Quite possibly the only stable time loop was one which involved a sufficiently creepy experimental result.
My pet theory was that Harry wrote "Don't mess with time" to himself because in a previous iteration, he had succeeded in using Time Turners to quickly factorise, and then parlayed this capability into a money-making scheme; shortly followed by a world takeover-cum-ascension to godhood, realised it was overall a bad thing, had one of his "this is that moment twenty years from then where I look back and point to exactly where it went wrong" moments, and used his power to return to that time, complete with the knowledge that he shouldn't ...
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.