Long time ago, while running cat5 in a large open ceiling, I stepped off of a large metal beam onto what I thought was the concrete ceiling of an outdoor awning, but was in fact droptile. Fell right through, of course, and landed more-or-less on my back. The floor was that hard institutional carpet.

I don't remember anything about the fall itself. After hitting the floor I immediately got to my feet, realized the breath had been knocked out of me, tried to call my partner's name, then sat back down. I think the pain came shortly after that.