The one fundamental blunder that led to all the other blunders that caused the current financial crisis

Outsourced to Joseph Stiglitz:

The truth is most of the individual mistakes boil down to just one: a belief that markets are self-adjusting and that the role of government should be minimal. . . . The embrace by America—and much of the rest of the world—of this flawed economic philosophy made it inevitable that we would eventually arrive at the place we are today.

This is from Stiglitz's article in the current Vanity Fair. For a summary, see this post on Mark Thoma's blog, which is where I got it.