Given how hard human bone is, I’m actually surprised this doesn’t happen more often in MMA fights.
If you’re at all squeamish, you probably don’t want to press play on the video below. In it, an MMA fight from Combat Challenge starts with one of the competitors attempting a low leg kick, a standard offensive attack. You can hear the smack of leg on leg as the taller competitor tries to raise his leg and block the strike.
As the fighter who delivered the blow takes a step back in his fighting stance, his right leg snaps in half and he falls to the ground. Needless to say, the fight was stopped. Again, the video is a little graphic so you may not want to watch.
The leg must have broken on impact with the initial strike and then just snapped with the slightest pressure on the limb. Take this as a reminder that kicking a man’s leg as hard as you can carries with it some inherent self-risk.