I know this is really late, but........................................ It isn't uncommon to have the situation Cliff described in either residential or commercial wiring. In the familiar 120/240v setup that most homes and many small commercial buildings have, there are two legs, each at 120 volts, that are exactly opposite in phase. Line-to-line voltage will be approximately 240 volts, and either line-to-neutral(or ground) will be about 120 volts. The neutral will carry the unbalanced current from two opposing 120v legs back to the source, in a multiwire branch circuit.
Electrical professionals should understand this, but it's not intuitive to the untrained person. This is why the NEC was changed a few years back to require that all legs of a multiwire branch circuit(one with a shared neutral) open simultaneously. As is usually the case, existing circuitry was not required to be changed. Of course, many residences suffer from uninspected "bootleg" electrical installations, as well, but that's a whole 'nother can of worms.
There is also a three-phase flavor of MWBC's, but very few of them apply to residential occupancies.
I hope this helps a little.
Dave