keep hitting stuff, and it will happen, only you will fracture the 2nd or 3rd metacarpal and likely need surgery. the injury that is being called into question is typically referred to as a boxer's fracture. it is a fracture of the 5th metacarpal neck, and it usually happens [guess how??] when people punch hard things (doors, walls, other peoples' faces).
most of the time, nothing is done except for a splint/cast for 4 weeks. sometimes it will need to be reduced (pulled on, and put back into place), but usually not, since there is a lot of motion in the ulnar sided carpal bones, you can tolerate angulation of the fracture.
if there is a rotational component at the fracture it usually needs to be pinned or else your small finger will rotate under the others when you make a fist. either way, its a stupid injury that almost ALWAYS involves your dominant hand, puting you out of work for several weeks