Here is a painful shot of one.
I was shooting a softball game, and I was switching lenses. I never do this, but this one time, I set this lens on the bleachers to grab a different lens out of my bag. A rogue pop fly foul ball comes sailing high over the fence and lands right on my lens like a meteor. (or more appropriately, a meteorite). The lens just exploded. Little parts and rings went flying. I have no idea if the actual glass internally was damaged, but I can't imagine that any sort of proper alignment is still intact. What are the odds that the one time that I actually set my lens down that a stray ball would fly over the fence right at that moment and land in that exact spot?
Needless to say, that was a painful lesson. I have since replaced the lens (it happened a few months ago), and I no longer let any of my gear sit anywhere but in my bag or in my hands.