Oh man I've got all you guys beat.
One time I was on the running machine and had it set fairly high (like a fast jog), when my water bottle vibrated out from the holder and onto the tread belt thing in front of me.
So I bent down to pick it up.
ops:
Bottom line is, my legs were taken out from under me, I did a faceplant onto the track, then was promptly flicked off into an exercise bike behind me.
It was like something out of looney toons.
So I got back up and kept running (of course) then the dude next to me said "err, are you okay? your cheek is cut open". I felt my cheek and thought "hmm, maybe I should stop now."
It's not the first time I've had to fill out a form for insurance purposes saying "the accident happened cause i did something stupid".