I like to say that the most dangerous people in a dojo are the intermediate belts because they're getting very strong and they know a lot of techniques but they don't yet have the control that more advanced belts have. Yesterday I was teaching an intermediate belt our self-defense techniques - a fun process whereby I get flung onto the wood floor over and over again. It went well, I got thrown maybe two dozen times without incident, except for that one time when I got dropped the wrong way and banged my elbow hard. The bruise only started to show 24 hours later. No damage, but it sure hurts.
(That's OK. I had to learn, too, and I'm sure I hurt the black belts who were teaching me way back when. It's all for the good of the sport. Plus it makes me look tough. Right?)