Liquidation preference is the number one way that founders end up with nothing. Works like this: VC invests $1m with a, say, 5x liquidation preference then if they company sells for, say $6m the VC gets the first $5m and the remaining $1m is split according to equity. http://www.gabrielweinberg.com/ has some really, really good articles on this kind of thing.
yes at $5m they will get nothing, but at $4m they won't be in debt.
5x is really a rip-off, but 2x (double-dip) is quiet common.
Liquidation preference at its core is an instrument to protect the investor. Imagine the following scenario:
An Investor gives you $1m for 50% of your company. A year later it sells for $1m (because it wasn't a hit).
The investor just lost $500k you made +$500k.