I've done the falling to death thing many times by dismounting certain toons while they are flying somewhere by accidently hitting my flying mount button. Usually this is three toons as I'm trying to catch the other two of them up to the same dailies in Icecrown so I forget about the three that are just hovering around. Best to put them in an inn somewhere until I'm done so they don't cast mount at all.

How about broadcasting your screen name and password in /say in the middle of a crowded city because I accidently left broadcast on (back when I was using Keyclone) and one of my toons had gone offline. After realizing this I quickly changed my pw.

I still do a few stupid deaths in instances as well, but I'm getting to the point where it's more rare, and this usually involves accidently leaving the druids bags all open so it's covering up the click spot where I'm trying to heal on my paladin's screen. Most of my heals are on macros now so that doesn't happen, but when I was first using repeater areas this happened several times, and I'd say to myself, "Why isn't the $O#% druid healing!"