I would like to disagree:
One day I was at the park playing one on one basketball with one of my friends. I had the ball, and he tried to reach in and steal it. The same moment he reached in, I moved in to try and drive it in, and I tripped, I'm not entirely sure how, but in any event, I bashed my knee into the concrete floor, and it started bleeding. This happened a few months ago, and the mark for it is still there. I didn't do it intentionally, it was by accident.