I have witnessed two very "unlikely" detonations. One happened to a World Champion IPSC shooter as he was unloading a magazine. He wound up unhurt and holding the bullet in his hand.
The other was an ejected case that struck a primer of a loaded round in an open box that was on the shooting bench. Since then, I make it a policy never to leave exposed ammo out when shooting semi autos.
Strange things can, and do, happen. I'm willing to believe a dropped box of ammo could detonate a single round.