Friends have the following situation. In investigating why a number of electric outlets seemed to go bad at their house, they found that a GFI receptacle in a bathroom disables many other receptacles throughout the house if the GFI is tripped. The GFI receptacle seems to be faulty, it is loose in the box and difficult to reset. Sometimes it must be jiggled before it can be reset. Naturally, they tried turning off the current to the GFI receptacle in order to replace it.
In the same bathroom, there is another receptacle with a night light on it. When they flipped breakers to turn off current to GFI receptacle, they noticed that no single breaker will turn off the current to the receptacle with the night light. (When the GFI receptacle is tripped, it does turn off current to the night light receptacle and many others.) Since the GFI has been reset and all the other outlets now work my friends paused their lengthy investigation to take care of other things in life and think about the problem .
The only explanation I see is that some mis-wiring that has connected the circuits from two (or more) breakers together. I suggest flipping pairs of breakers to see if a pair of them will turn off the current to the night light receptacle. That could reveal which circuits are connected. Is that a good first step?