My one GW Reuben used to play so hard that he would "bruise" his beak. I would notice that he would favor one side and wouldn't attempt to bite me. I had to feed him soft foods for a few days and then he was fine.

Zen also has a bruise on her beak. Beaks are similar in makeup to human fingernails - if you bruise your fingernail it has to grow out, same with a beak. Zen's has become progressively lighter since it's been healing. It started as a black spot and now is brown and yellow.