2 times I can think of.
1) Had a lady riding me Cow Girl very vigorously once. When she went up, I slipped out of her and then she came crashing down and damn near broke my penis.
It still hurts to think about.
2) Was going at it with one of my ATF's (retired now

). We were doing it in the kitchen, with her on the counter facing me. Well, I guess we had things shaking pretty good because in a flash this big, heavy white porcelin bowl comes tumbling out of the cupboard above us. The door swings open, hits her in the back of the head and the bowl falls on me giving me a bleeding nose. So we're both laying on her kitchen floor, naked, me bleeding, her dizzy and broken porcelin all over the place. We spent the rest of the afternoon laying on her bed with ice packs and drinking tea.
We laughed about it on many occasions after but at the time it was pretty painful.
**Good thing I'm not a married pooner or I'd have had to come up with some kind of story to explain this to my SO. I told my friends and employees I got hit with a puck playing hockey.