I sprained my ankle the first time I tried to go from chin on seat riding into seat drag. It wasn't a very bad sprain though, and I actually managed the transition probably less than a week later.
I sprained my little finger doing feet out hand wheel walk. My finger got caught on the wheel somehow, and was very sore for several days.
Thankfully, I haven't broken any bones yet, although I have broken lots of unicycle parts.
