Just about two years ago, the Reserve Primary fund, housed at one of America's oldest money-market operations, broke the buck. That is, the assets in the fund took a nosedive, to a value well under $1.00 a share. Money markets strive hard to maintain the $1=1 share relationship (any interest or capital gains on the assets in the fund are what gives the money market its yield). That's what makes these funds look very similar to a bank account: you always get out at least what you put in. When a fund breaks the buck, it's as if Citibank suddenly told you that your savings account only had $587 in it instead of the $900 you deposited.
As with bank accounts, losing the money you deposited is now so rare that when it happened, people panicked. The ensuing run on the money markets was arguably the worst moment in the financial crisis, and like many people, I believe that this, more than any other event, irrevocably committed the government to massive intervention in the financial sector.
I've been told by multiple sources that even before the Reserve Primary debacle, a number of firms had been quietly "topping up" their own money-market funds to keep them from breaking the buck. Indeed, as I understand it, the only reason that Reserve Primary broke the buck was that it was a small firm catering mostly to institutional investors; when the commercial paper markets went haywire, there was no parent company with deep pockets to keep it from going under.