Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread:
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
I now imagine that it is. Here is my scenario, with so much detail that its probability is extremely small, so that it cannot be a spoiler:
Dumbledore, being cleverer than in canon, discovered the existence of the diary Horcrux. After diligently searching for basilisk venom and researching any other safer means of destroying a Horcrux, he realised that he would have to use Fiendfyre. He broke into Malfoy Manor but found it more difficult than he expected and was badly weakened when he found the diary, so he was unable to overcome additional protective enchantments on the diary itself and remove it. Having good reason to believe that the manor was empty, however, he used Fiendfyre right there and then used his last strength to escape. As it turns out, Narcissa was home, and her valiant efforts saved the manor from destruction but cost her her own life.
Later, Dumbledore spoke to Lucius to apologise. While he did not dare to explain why he had started Fiendfyre in the Malfoys' home, he told Lucius that he never intended to kill anybody and only reluctantly cast the spell that would have destroyed the house. He also told Lucius that, while he could not give details, the act was only necessary because the Malfoys were still working for Voldemort, and he warned Lucius to rid himself of anything to do with Voldemort to avoid any future accidents.
When Harry discovers these facts, he will realise his omission and beg Draco to trigger the first condition. Drama ensues.
On second thought, the character of the Dumbledore above is more like in canon than in MoR. Under the circumstances, Dumbledore would probably start the Fiendfyre even if he knew that Narcissa might be killed. This is another missing exception: if the act that killed Narcissa, despite her having no dirty hands, saved more lives than it took. (Whether or not that applies in this case, it would certainly be Dumbledore's defence, and Harry might buy it, given enough evidence.)
It would be very interesting to watch Harry try and convince a very smart and angry Draco to let him out of his promise, in such a situation.