Fractures of the 4th or 5th metacarpal resulting from hitting someone in the jaw are so common that they have a name: "boxer's fractures". When I trained at Gleason's Gym it was made clear that getting into scraps outside of the gym was a good way to get thrown out of the gym, as no trainer wanted an up and coming boxer to be injured outside of the ring.
In my Chinese martial arts training hand conditioning was de rigueur. I can still punch a hard surface (e.g. a plaster wall, tree, lamp post) with no negative effect....but it took a lot of training to get to that point.