From Chapter 27:
Or if you were a perfect Occlumens, you could race ahead of any probes, answering queries as fast as they were asked, so that the Legilimens would enter through your surfaces and see a mind indistinguishable from whoever you were pretending to be.
Even the best Legilimens could be fooled that way. If a perfect Occlumens claimed they were dropping their Occlumency barriers, there was no way to know if they were lying. Worse, you might not know you were dealing with a perfect Occlumens. They were rare, but the fact that they existed meant you couldn’t trust Legilimency on anyone.
So, if Harry wants to prove he's an Occlumens, he can. But if he is intending to lie under Veritaserum as part of a framing plot, he doesn't want to prove it; he wants to lie. And if he's gotten good enough at Occlumency over the last 50 or so chapters, he might be able to pull it off.
It might be too late for that. He told Draco that he's an occlumens, and it's highly probable that Lucius questioned Draco under veritaserum. Draco may already have told him, and if he hasn't, the fact that he could could undermine any plan hinging on it.
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: