Our home in the US was re-wired shortly before we bought it, within the past ten years. I was working on one outlet (call it outlet A) to possibly add an outlet nearby
and had turned off the power to it at the breaker. The power remained on at a nearby outlet B with an adjacent breaker in the panel (but the breakers are not tied together).
I tested the outlet I was working on and found no voltage between the hot and neutral, hot and ground, or neutral and ground. On disconnecting the neutrals from the screw terminals, I was startled to see a spark. The voltage tester lit up when put across the two neutrals at the screw terminals. This voltage disappears when the circuit for outlet B is also turned off.
The box for outlet A has three strands of Romex coming in. The receptacle has two white neutral wires at the screw terminals and a third pushed in to the back of the receptacle. It looks like the third hot wire is tied in with the others using a wire nut.
I wonder if a shared neutral situation between the circuits for outlet A and B could explain the spark I saw, or if there is some other sort of legitimate wiring scheme that would explain this, or if we should get an electrician out to get it fixed.