In 2006, the original money market fund, the Reserve Primary Fund, decided to purchase risk-free commercial paper, and by 2008 over half its assets were in this risk free, high quality commercial paper.

Unfortunately, a significant part of this risk free, high quality commercial paper was issued by Lehman. And that risk free, high quality commercial paper turned out to be worthless, causing the oldest money market fund to fail and forcing it to liquidate, at well below $1.