Being a biker and an ex climber, I have several...
Two that spring to mind:
A friend and I were asked to help out on an outward-bound course teaching a group of kids to abseil. A girl of about fourteen got a case of the fear halfway through her first descent from a sixty-foot scaffold. I stepped through the guardrail at the top of the tower and gently talked her down. As I slid back under the rail, my hand automatically swept behind me to lift my rope clear so that I didn't trip over it. It wasn't there. I'd been leaning over the edge of the tower with the safety rope in my hand attached to a panicking teenager and I'd forgotten to clip myself on to my own rope. Ironically, we had incessantly drilled one rule into all our students: "What is the most important knot? Your own". Close enough for me, thanks.

Ice-climbing in Glencoe, second man up a waterfall following a nice route with protection all set. I felt a shudder, and sensed myself falling. Standard practice is to flatten yourself against the ice to reduce the leverage on the axes and crampons and this is what I did. Unfortunately, with my face pressed hard to the ice, I could see the scenery to my side still creeping upwards - the sheet of ice onto which I was firmly anchored had broken away and was slipping down the mountain with me on it. It stopped after about twelve feet. No tunnels or lights, just a nice warm feeling as the contents of my bladder ran down my leg.
( L-Space - scientia sapientes vocat., Mon 29 Nov 2004, 11:21, Reply)