Now I have seen everything.

[kwired]

Very strange because there is no dining room heater. There is also no living room heater but there is a 2 pole breaker labeled such with a 12-2 UF cable connected to it������

Probably was a 240 volt heater at some time and someone not knowing exactly what they were doing removed and re used the line for a receptacle - and somehow interconnected with another existing circuit. When GFCI isn't tripped, you should have at very least a line to neutral fault and trip at least one of the breakers, and likely the GFCI at the same time.

I had a house with a fuse panel one time that had a fuse that apparently had a short circuit/ground fault because whenever you put a new fuse in it instantly blew. Was somewhat newer house then most with fuse panels, but still built in 50's or early 60's. Thing was even with this blown fuse there didn't seem to be anything in the house that didn't work.

finally found that circuit was mistakenly tied to another circuit in a box somewhere supplied by the other 120 volt line, so any time you put in a fuse you were closing a 240 volt line to line fault.

 

[growler]

I removed the nearest dining room outlet to see if that UF was connected to the outlet. It was. Black on the dark terminals and white on the light terminals. WHY WOULD ANYBODY DO THIS?

I would think they were changing out the receptacles. A 240V 20 amp receptacle looks a lot like a single 120V 20 amp receptacle ( well, I'll just put a duplex outlet here).

I think this because you said it was marked for dinning room heater. It may have even been a 120V dedicated circuit at one time and they decided to get a bigger heater ( easy change to make).

Many times you find things that make no sense at all so you can only speculate as to the cause.

 

[kwired]

I would think they were changing out the receptacles. A 240V 20 amp receptacle looks a lot like a single 120V 20 amp receptacle ( well, I'll just put a duplex outlet here).

I think this because you said it was marked for dinning room heater. It may have even been a 120V dedicated circuit at one time and they decided to get a bigger heater ( easy change to make).

Many times you find things that make no sense at all so you can only speculate as to the cause.

That is understandable, but this goes a step further and somehow ties into another circuit. Since it was on the load side of a tripped GFCI it feeds 240 volts to everything on load side of that GFCI. Reset the GFCI and you have fault conditions, exactly what depends on exact connections, but you do have a line to neutral fault for certain and a 50-50 chance of line to line 240 volt fault.