Madam Bones killed Narcissa. "Someone would burn for this." pg. 879 of the pdf.
EDIT: This is likely the double Taboo tradeoff. The original tradeoff was going along with the innocent Narcissa being burned as a war tactic or just covering up for the Aurors and Bones after they did it because he needed them - he was protecting his pieces for the larger war. Either way, it probably served his war aims to take credit for the attack and have Malfoy believe he was ruthless enough to do it. Mutual Assured Destruction.
The new taboo tradeoff is much the same - he's protecting the same people, and now himself as at least an after the fact accomplice, by letting another innocent be tortured to death.
Why, then, did Dumbledore take the initiative to cover for her? To protect her from Lucius' revenge?
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now 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.)
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, seven, eight, nine, ten.
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: