Many of the people I've talked to about the financial crisis like to suggest that the cause for the financial crisis was the change in lending practices of Fannie Mae during the Clinton Administration (1999). Although these changes did increase the risk on the balance sheet of the largest mortgage lender in the US, I don't feel this played a significant role in getting us to where we are now.
The reason, because in 1999 and until April 2004, the amount of capital that could be leveraged by investment banks was capped. However, this rule changed on April 24, 2004 for any investment bank over $5 billion in market capitalization. Banks who qualified were now allowed to almost double the amount of leverage. Who qualified under this little known rule change? Just 5 companies.... maybe you remember them: Bear Sterns, Lehman Brothers, Merrill Lynch, Mogan Stanley, and Goldman Sachs.
So before the rule change, the fuel that could be put on the fire was at least known and limited, after the rule change, the fuel [read potential disaster] almost doubled.
This is a slideshow by the NY Times. What's great about this is the actual audio from the meeting when this rule change was debated and passed.
"The Day the SEC Changed the Game"