I feel I could post for over 95% of the internet right now. The **** I see at the gym...
As for myself, I remember having just sat down on one of the benches at the jungle gym machine, as I call it, to do seated rows with a wide hammer-grip bar. God knows where my head was, I retracted my elbows to bring the bar towards my chest, but I hadn't extended my legs yet, so I pull the bar right into the funny bone of my knees. Made me scream so loud and practically having a seizure.
EDIT: oh actually the post above reminded me of a moment a couple years back when I first started lifting, and I would explode the bar up (more than I do now, even). I was doing incline bench, starting a warmup set, and on my way up I accidentally hit the bottom of one of the uprights so hard the bar bounced and hit me on the head, practically knocking me out.