Installed incorrectly how?
The breaker box has metal "fingers" that are supposed to press in between the contacts in the breaker. The breaker on the left still has it's contacts. The finger slides between these. The finger for the seriously torched breaker was bent up and instead of being pressed between the contacts it was laying on top of the contacts. Good enough for an electrical connection, but obviously not a good connection. This last trip was down in Florida, and it was hot, so the camper had two A/Cs going, some fans, etc etc. A/Cs ran pretty much non-stop for a week.
Bad connection, high draw, excess heat, melted breaker. Surprising thing was it was actually functioning right up till I shut it all down at the post. I can't believe it never popped. When I pulled it out it was hot as a firecracker, and the contacts dumped out into the palm of my hand. My first thought was "chinese piece of junk", but turns out it was a Cutler/Hammer breaker and last I knew they were quality pieces.
I bought a new breaker, bent the finger back where it should be, popped that sucker in, and it ran cool as can be for the rest of the trip. Another week of 24/7 A/C and fans, etc and it never got beyond room temperature.