Group sex (one couple, one single guy, one single woman and me) a good few years back. One of the women there had never met me but my... reputation preceded me. After a while she just says 'Damn, I have to try that!' and pulls me over, so I grab a condom.
I hadn't 'found my brand' at that point and a combination of a too-small rubber, an amazingly toned set of muscles on her behalf and a little over-exhuberence on both, meant that on one out-stroke she stripped the thing straight off me. I feel a little 'pop' as the end jumps off and it is gone! OK, so that is a little embarassing.
What is worse is that I can't get the damn thing out! It's in there, somewhere! I'm trying to be gentle (don't want to go hunting around like I'm trying to find a bolt I've dropped when working on the car!), she is giggling, everyone else is gathering to watch... and I can't find the bloody thing! Eventually the other woman there (with a better grasp of interior female anatomy that I had at the time) says 'Let me have a go' and after about 30 seconds retrieves the wayward prophylactic. Didn't seem to dampen anyone enthusiasm, at least after they'd all mocked me mercilessly!