JennM;1002398 wrote: Yes, I have. Any idea how it happened?
If it heals without infection, and the fish still has the ability to eat, it will be fine, however disfigured.
I had a school of Mono argentus that came with my 1000g tank back in the day. We had them in a 75 while the big tank cycled, and some idiots came into the shop and were getting a kick out of startling the fish, causing them to dart in all directions, and one collided with a tank decoration, and shattered its lower mandible - skin was broken and the fragile bones kind of hung off for a few days til they fell off.
I was afraid I'd lose the fish, but it healed up, and despite having a permanent "surprised" look on its face (we called him, Mr. Bill - LOL- OH NOOOO!) it ate fine and lived another 4+ years and I sold it and the rest of the school when I sold the tank.
Chances are your goby suffered a mechanical injury of some sort - as long as it can still eat, it will survive.
Jenn