Bad creaky FSA Gossamer crankset

Exactly the same that happened to me... I just had no idea at the time what the cap coming loose meant. If your crank arm's actually been tightened enough you might not have the same problem I had. (crank arm came off on a fast downhill)

This happened to me too.

I still have the scar on my hip.

I heard a tinkling sound behind me and thought I'd dropped something. Stopped, couldn't see anything so carried on. Got out of the saddle for a hill and crashed down hard as the LH crank arm came off.

The direction of rotation works it loose. As it rubs your shoe it spins out quite quickly. Even if checked at the start of a ride it can be out by the end.

The fact that every website sells spare ones for a few pence tells me it happens a lot.

I've loctited mine in now.