Once more we have a combination of high resistance, perhaps some corrosion on the blade or receptacle, and an overloading of this circuit well beyond the safe range.
This boat was running three portable AC heaters, water heater, a reverse cycle heat/AC unit, a toaster, microwave and refrigerator all on this single 50A circuit.
When I got there he was pulling 42A through a 50A service and the plug measured 389 degrees, at the melt point, in February.
Because of this boats poor insulation it was not uncommon for all FOUR heat sources to be on all at once....
According to the owner he used to trip the main breaker "a few times per week" it had not tripped the day I found this.......