Analysis of "What’s 2/3 of the Average?"

I'm honored to be cited here. I heard about Y Combinator on EconTalk. I had no idea it was a place to find links to interesting posts and news.

Anyway, this post is applicable to investing and bubbles. How far from common knowledge is assumption about rationality? If it is sufficiently far then it is rational to buy/sell at a price very different from the one that would obtain with common knowledge rationality.