You've got to be pretty unaware of things for the crankarm to fall off. It's going to be loose and wobbling for awhile before it falls off.

The two bolts that actually hold the crank arm on to the spindle have to work themselves loose. Then the bolt on the end that pre sets the bearing load has to back itself out a good half inch.

This doesn't happen in one pedal revolution.

Admittedly, properly torqued, and assembled with the correct Locktite, recommended by FSA, on the spindle and on the bolts, and this shouldn't happen. (Red Locktite 641 on the spindle, Blue locktite (like in the picture) on the bolts.)

But if it starts getting loose on a ride, it's a simple fix with a mini tool.