Wow, I just had a shocking experience
I am working on an in wall touch screen (which is another story for later), and was going to pull power from an outlet below. I believe that outlet is on the same circuit as the outlet where I ran the neutral to the switch that was missing it.
Well, I shut power to the outlet I was working on, and confirmed with a meter it was off. So I pull the outlet out and start disconnecting the wires. I was disconnecting a neutral and BAM, a nice little bite. At the same time the kitchen lights dimmed and came back on. I was totally baffled but my 'buzzing a/c detector' does pick up slight current on the neutral line! If I turn off the kitchen lights the buzz stops.
It must be that with the light on there is some current going through that neutral! Wow, tough way to learn that lesson. Guess I need to turn off kitchen light circuit too. Just weird cause I never got bit from a neutral line before!
So I guess there is a danger to pulling a neutral from another circuit after all. But I guess it is ok as long as I am not messing with that circuit.