So I go over a property that belongs to a friend (rental) because the tenant was complaining about the bathroom gfi not working. So I get in there and it's one of those gfi, switch combos. The switch controls the vanity light and is operational. I pull out the device to find one piece of 2 wire romex (old cloth kind) coming into the box. My first thought is the switching is being done either in a hidden j box in the wall or the fixture box for the vanity light and some idiot threw in the gfi combo deal to satisfy an inspector that never bothered to test an outlet. I wish I could remember the exact configuration of the wiring on the device but I can't. I know the ground was connected to the neutral screw and the white wire to the hot screw of the gfci, and the black wire to one of the little wires that comes out at the top. Just to make sure of my theory I do a continuity on the black and white wires, and my thoughts are confirmed. So then I test voltage between black and white and I get 120!????? So I pull the light off and it appears to be what I think with the power coming into the fixture box, going down the black and coming back on the white to the light. So I began to investigate further into this box (there were more splices) but the tenant starting raising hell (it was late, I came after work) so i put the light back, installed a regular switch and told them I'd be back on Monday. So I'm thinking a back feed issue but still I'm confused how across the 2 wires is 120 yet I put them together and the light turns on. Any ideas??