This happned to me when I was drunk and working on my bike. I took the bike for a test ride and forgot to put the crankbolts back in past finger tight. About halfa block down the street my left crank falls off onto the road and I realize I'm an idiot. It was ISIS splined too so the splines on the bb and cranks are totally mangled. It wasn't a huge loss though. I also discovered the right crank bolt on my speedy bike comes loose after a few weeks. I was told this is because I'm doing trackstands on it but it also happened constantly on my BMX which is freewheel and ridden by my room mate who has no skills.

The problem with crank bolts is its really hard to tell how hard to torque them.