Probably when I did something entirely stupid to win a race. It was an obstacle course and I threw myself over this slide thing, however, I landed with my chin on the ground in a "U" shape with my legs behind me. If my back wasn't so flexible, I probably would have broke it... It's never been the same though, I must have crushed some vertebrae or bruised it if possible (I didn't go to the doctor, because I felt too embarrassed to tell my parents that I'd done something so idiotic while away from them), I couldn't lean backward for about 3 months.
My other thing was when I was dancing in modern for a dress rehearsal, and we had to wear these long flowing skirts with no tights. Well, The part in the dance had me and another girl run and slide on our knees under some other girls dancing... I ran to do it, and got on my knees to skid, but my skirt was not underneath me. Skin skidding across the floor- OUCH.....
I got this horrible cut all down my knee. It was a ten minute dance though, so I just had to keep going and I had nothing to clean it with for those couple hours I was there. It ended up getting infected. It was pretty nasty.