High school gym class was playing tennis. I'm not too bad at tennis, but I was goofing around and really shanked a serve that went directly to my right. Skipped once and then bounced up into the crotch of the guy playing the net on the next court over. Hilarity ensued as he rolled around on the ground moaning.
In fact, a lot of my funny sport moments involved the crotch being hit. Reminds me of when I was on the high school soccer team and played in a game during some rain. No lightning, so play continued. Thing about soccer in wet grass though, the ball doesn't really bounce but instead skips like a stone if hit with some force. So I went to bounce a goalkick with my chest to the ground, but the ball skipped and went a bit lower than I had expected. Ball nails me in the nuts, and hilarity ensued as everyone else, including the ref, laugh their ass off while I rolled around the wet grass, calling for the trainer.
I've got more, but I'll let others contribute.