The taboo tradeoff is presumably Lucius being asked to trade off his chance at revenge.

What happened here?

The Veritaserum was brought in then, and Hermione looked for a brief moment like she was about to sob, she was looking at Harry - no, at Professor McGonagall - and Professor McGonagall was mouthing words that Harry couldn't make out from his angle. Then Hermione swallowed three drops of Veritaserum and her face grew slack.

Oh, thank you, that's it, that's the answer: Bellatrix is Narcissa's sister, and of course Lucius would be more comfortable blaming Dumbledore than Bellatrix, not only for family reasons but for fear of Voldemort.

Plus, consider the Law of Dramatic Efficiency: Bellatrix is one of the few people we've met who would fully trigger Harry's oath (to take Narcissa's killer as an enemy) yet Harry wouldn't want to kill. Because Bellatrix wasn't "tricked" into killing Narcissa. Brainwashed, yes, but not tricked.

Bellatrix meets all the conditions for Narcissa's killer:

1. If it's not Dumbledore, it has to be someone Lucius would rather not name to Draco. Bellatrix: sister-in-law and Voldemort's chief lieutenant.

2. It has to be someone Lucius has been in no position to take revenge on in the intervening years. Bellatrix: in Azkaban.

3. It ought, dramatically, to be someone within the oath yet very uncomfortable for Harry to go after. Bellatrix: in Harry's mind, brainwashed into her evil, but not tricked into the murder of Narcissa.

So Bellatrix fits perfectly. Lucius blames Dumbledore, knowing Draco won't trust Dumbledore claiming the contrary, and knowing how dangerous it would be for Draco to go after Bellatrix -- or for Draco not to accept Bellatrix as an ally, if Voldemort returns.

Lucius lied about the killer so that Draco wouldn't want revenge on someone so unsafe to want revenge on.

Voldemort himself is another one Lucius would lie to Draco about, but Voldemort would probably not have burned Narcissa alive, and it doesn't have the storytelling punch, because Harry has far less problem taking Voldemort as an enemy than Bellatrix.

So I think you've got it. It's Bellatrix, and Lucius lied about it to keep his son from a dangerous revenge, and Harry will have a huge problem once he finds out.

So, what happened to Narcissa?

Good point. My interpretation of what you're saying is that the error is actually failure to re-plan at all, not bad math while re-planning.

There's a phrase that the tech world uses to describe the kind of people you want to hire: "smart, and gets things done." I'm willing to grant "smart", but what about the other one?

The sequences and HPMoR are fantastic introductory/outreach writing, but they're all a few years old at this point. The rhetoric about SI being more awesome than ever doesn't square with the trend I observe* in your actual productivity. To be blunt, why are you happy that you're doing less with more?

*I'm sure I don't know everything SI has actually done in the last year, but that's a problem too.

There's a phrase that the tech world uses to describe the kind of people you want to hire: "smart, and gets things done." I'm willing to grant "smart", but what about the other one?

The sequences and HPMoR are fantastic introductory/outreach writing, but they're all a few years old at this point. The rhetoric about SI being more awesome than ever doesn't square with the trend I observe* in your actual productivity. To be blunt, why are you happy that you're doing less with more?

*I'm sure I don't know everything SI has actually done in the last year, but that's a problem too.

I'm usually confused by these kinds of comments. Reflections on rationality a year out had even less content and was less well-written and less detailed, but was massively upvoted. Kevin hit the nail on the head with his comment on my Existential Risk post:

I'd like to point out some lukeprog fatigue here, if anyone else wrote this article it would have way more points by now.

Also, this is a personal contribution to the long-running "What good is rationality?" discussion on LessWrong.

What makes you think there's much better to be done? Some games or problems just aren't very deep, like Tic-tac-toe.

The Ants problem -- if I'm understanding it correctly -- is a problem of coordinated action.

One of the interesting aspects of the winning entry post-mortem is the description of how dumb and how local the basic strategy the winner used:

There’s been a lot of talking about overall strategies. Unfortunately, i don’t really have one. I do not make decisions based on the number of ants i have or the size of my territory, my bot does not play different when it’s losing or winning, it does not even know that. I also never look which turn it is, in the first turn everything is done exactly the same as in the 999th turn. I treat all enemies the same, even in combat situations and i don’t save any hill locations.

Other than moving ants away from my hills via missions, every move i make depends entirely on the local environment of the ant.

Interesting reading, overall.

EDIT: Another example of overthinking it: http://lesswrong.com/lw/8ay/ai_challenge_ants/56ug One wonders if the winner could understand even half those links.