A fascinating and infuriating article on Wired that tells how a mathematical formula contributed to the market meltdown. It's a lesson in looking for the easy path and the power of the mentality of crowds.
The part that I'll remember the next time this happens (and it will, eventually):
In financial markets, everybody doing the same thing is the classic recipe for a bubble and inevitable bust.
Wired: Recipe for Disaster: The Formula That Killed Wall Street