Ha ha ha! Now I get it!
.....
Well, it was pretty lame.
... But seriously... When approaching a home older than say, 1980, be really careful with electric wiring.
One thing I discovered the hard way is the concept of sharing neutrals. This is even more common in conduit systems, but can also happen with Romex.
The problem is that somewhere along the line, two circuits get tied together on the neutral. You are playing elsewhere, and have the breaker/fuse off. It looks good, even with a non-contact tester - no energy. Then you open up the neutral connection, and suddenly, get shocked. Basically, you can get return current from the tied in neutral if a light is on somewhere.
Oh and then there are the neutral ties to ground. Oh sheesh. I encountered that recently too. Basically using the ground wire as a neutral return. I should have taken a picture of this switch box I opened. It was a real sh.. show.
Nothing wrong with sharing the Neutral between two circuits
on different phases. It's done all the time in the Chicago area, and we have very strict electrical codes, conduit everywhere in residential. In fact, it's advantageous - with those two circuits at max current (say 15A), the current in the Neutral cancels out, and has zero current. There can never be more than the max current on the neutral, and usually less (assumes resistive loads, changes a bit with reactive loads). So at max current, instead of 2 neutrals getting heated up a bit with 15A current each, you have zero current and zero heat in the single neutral. It flows directly from one phase to the other.
The issue you describe with having power from the other leg when the leg you are working on is switched off - I had that happen to me, suddenly the lights got super bright in the room, they were picking up current through the other leg, and one leg had more stuff ON than the other, so it got more than 1/2 the 220V combo of both legs.
Now, this won't happen if you simply disconnect the neutral
from a device on the OFF leg. But if you
BREAK the Neutral (which is easy to do if the device connects to a Neutral that comes in and out of the box and are tied together on a single wire nut), then you can have the situation you describe, current on the ON leg can flow through that disconnected Neutral into the other leg - whoops!
Considering that, it is a bit surprising that it isn't mentioned to turn off both breakers on a neutral. And I'm trying to remember if the correct way to wire a shared Neutral is to have a separate pigtail for the device Neutral, so you can disconnect it w/o breaking the downstream Neutral connection - but I don't think it is done that way?
Oh, Neutral connected to Ground anywhere other than a single connection at the panel is a clear NO-NO!
-ERD50