Appearantly it didn't give a good gift for Christmas or had too much egg nog and started a fight and received the sore end of the deal. Ha Ha Ha
No seriously, I read where it could happen by bumping it on a rock. From what others have said... just keep the water quality high and it will heal its self. One recommandation is to use a dose of ebsom salt.
I am no expert but that is what another forum said to do.