So this guy I know calls me Saturday morning...
He was simply changing out a single light fixture in a bathroom. He shut off the switch on the wall and removed the light fixture. When the fixture was removed other lights in the house started dimming and getting brighter and his doorbell transformer started humming. It was then he flipped the breaker and called me.
I asked if he had a meter, which he didn't, so I took a ride over. With the breaker off he had power at the neutral wire. Figured it out yet... I pulled the cover off the breaker panel and sure enough, black-red, black-red, on every circuit. I told him that his breakers on these shared-neutral or edison circuits should be tied together. I also told him that one of his white wires was broken or disconnected on one of those two circuits.
So then he tells me that everything worked fine before he removed the light. So I look at the old light and whoever had put the light in originally wired the neutral and the ground together. Either that or the problem occured in the past and someone tied the neutral to the ground to fix it.
Luckily for me this was a guy I know and not someone I felt obligated to spend my weekend with tracing wires. He wanted to put the new light in and just wire the neutral and the ground together like the old one. I explanied that he would then have a potential shock hazard on every switch and outlet cover screw because the ground wire was carrying current. He tells me he is going to try and find the problem, I tell him to call an electrician and change the batteries in his smoke detectors.
To make a short story long he spent all of Saturday and most of Sunday messing around until when he flipped the switch in the bathroom the hallway lights came on. He called me again, and again I said to call someone. Surprisingly he found an electrician to come out Sunday afternoon. The electrician traced it to the main feed for the two cicuits and ran a new 3 wire from the panel to the first junction box- problem solved for $550.
Sad part is he had an electrician replace his panel five years ago and the guy that replaced it did not tie the breakers together. So even if he would have shut the breaker off before removing the light he still would have had a hot neutral at the light fixture.
Be careful out there.