I have two that come to mind, the first rather more distant and less traumatic than the second:

1) Riding alongside a friend down a quiet alleyway, me on my gaspipe Puch, he turns to me and says "I tried to switch hands while cycling, and I fell off". What do I say? "No way, that wouldn't happen"... I don't blame him for laughing when I flew over the bars into a crumpled heap on the concrete

2) Couple months ago, I just finished my final university exams and could finally get riding again. The chain on my (fixed) 1935 Raleigh Sports looked a little slack, so I flipped the bike and pulled the wheel back. Spinning the chain, it still looked a little slack, but I wanted to make sure. Touched the chain while it was still spinning at speed, realised exactly what was going to happen before it did. My finger got dragged into the mech, impaled on the fixed cog and bent around the spokes. Had to be cut free and needed a 2 hour surgery. The bone completely shattered and the top half inch of my finger was hanging on by a thread. They saved it, but it does look a bit of a mess. Still can't use the finger, but the Raleigh is back in service (and now has a freewheel!)