Dave was recently published in an article in CNBC regarding money market funds. In 2008, money markets fell from $1 to 97 cents - this is referred to as "breaking the buck". Money markets are viewed as a "safe" investment, but there can even be times when money market funds experience volatility and negative returns. Markets aren't always rational, and it is important to define "safety", especially during low-yield environments when investors are tempted to sacrifice quality in order to get some return. If you truly want a cash reserve, it might be better to place those assets in a traditional banking account.
There is a difference between making an investment and expecting a return for it and setting cash aside to protect it. In summary, the more you pursue return, the more you take on risk. It is important that before making an investment, you define it's purpose and what you are expecting from it.