Similar to the old glitch with multiplayer signs where if you broke them but weren't allowed to, it would erase the words on the side client-side only.
If you break an edited mob head but aren't allowed to, it changes to the head that was edited. Simply hitting the head once fixes it.
Example:
A skeleton skull had been edited to look like a player's head. I broke said skull with my fist, but wasn't allowed to destroy blocks in the area I was in, and it looked like the regular skeleton skull. I punched it once and it turned back to what it was.
Sorry, Java is being derpy and I can't add screenshots. (Mac problems maybe?)