That sorta happened to me once on my road bike when I had the single sided spd/platform combo pedals. Foot made contact with the wrong side, slipped off and my tail bone hit the saddle.. HARD. It hurt for a month and I had to sit sideways. Probably bruised..

I can trackstand, but not for long, so if the light's not gonna change for a while (you can tell here by looking at the walk signal) then I don't bother.