When I went zip-lining a few years ago I got stuck about 300 feet out on a line with a 220ft drop below me, and part of my harness got unbucked, and I had to hang there for about 6 minutes before someone came and helped. If I had let go, I might have slipped but then again I might not have.
It was a shame though, the view was beautiful, and there was nothing but mountains and trees in sight. Sucks that I had to see it when I was holding on for dear life.
OR the time fell out of a raft on a huge waterslide, but that was actually fun and scary so I guess it doesn't count.