Me and my twin sister played Varsity fastpitch softball for our high school. I was centerfield and she was shortstop.
Weeeell, there was a high fly ball, and from my position I could tell it cleared the dirt, so I was running forward, calling the ball "mine! mine!". Unfortunately, my twin sister was also calling the ball and running backwards.
Needless to say, we didn't hear each other, and although it should technically have been MY ball, heh, we collided and fell to the ground. Once we could breathe again (we were two of the fastest on the team), the trainer came out to check to see if we were hurt. When we hit each other we were both bruised, but the angle that my sister hit the ground at caused her collarbone to SNAP.
Aaaand I felt absolutely AWFUL about it. she was out for about half the season. X_x