At the end of class, we were doing hai sun do (sp? that back stretch where you get underneath someone's center and bend forward and arch your spine, so they're lying on your back). I started stretching out a tall, heavy ex-bodybuilder who had just started practicing with us. No problem, I thought - I've stretched out people heavier than that before.
What I didn't realize was that his center of gravity was higher than usual. As I bent forward, he slowly started sliding forward down my back, and because of his weight I couldn't lift him up again. While horrified visions of him taking the fall badly (he was new, remember) flashed through my head, he just calmly said something like, "Oops, lost it" and rolled down perfectly.
All this in front of sensei and the rest of the students, who had finished up already. Boy, did they all laugh their a**es off at me.