[myg204]

Swimming.

[stordoff]

No threat of falling, perhaps, but open-water swimming (for example) isn't risk-free. My mother has hit by a boat during an open-water race, and was very lucky to avoid a spinal fracture.

[pimlottc]

You've never hit your head on the end of the lane while doing laps? Especially backstroke...

[kasey_junk]

I gave myself a nasty concussion during a mistimed flip turn when my goggles had filled with water.

I suspect I'm in the extreme minority though.