i always enjoyed killing a chain when riding and smashing your knee on the stem. or just eating it.
one of my last races i was going through the six pack, about to hit the waterfall in the 2nd to last corner, and some dude clipped me in the air. not fun.
the worst thing was always going out street riding, and busting an axle no where near home… i got wise and started carrying a spare in my backpack.( this was before the days of 14mm axles)