I may be pointing out the obvious, but...
Professor Quirrell closed his eyes. His head leaned back into the pillow. "You were lucky," the Defense Professor said in a soft voice, "that a unicorn in Transfigured form... did not set off the Hogwarts wards, as a strange creature... I shall have to... take this outside the grounds, to make use of it... but that can be managed. I shall tell them that I wish to look upon the lake... I will ask you to sustain the Transfiguration before you go, and it should last long enough, after that... and with my last strength, dispel whatever death-alarms were placed to watch over the herd... which, the unicorn being not yet dead, but only Transfigured, will not yet have triggered... you were very lucky, Mr. Potter."
This is how the troll was smuggled into Hogwarts without the wards going off. In all likelihood, Quirrell had the transfigured troll on his person when Dumbledore identified to the Hogwarts wards "The Defense Professor stands within this circle". Trolls self-transfigure as a form of regeneration, so the transfiguration would not kill the troll or be detectable.
I'm not sure he'd needed to do that. Until we hear otherwise, he has access to all the knowledge of Salazar, who knew enough to build Hogwarts. Which also means the source code to the wards and the means to change them.
Can you even transfigure something that transfigures itself back? Of course Quirrell can do it if it's possible, but is it possible?
New chapter!
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 102.
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.)
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: