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.