Well, a service call we had to make ourselves due to some creative wiring, back when I was a kid of 6: We had rented a new apartment with 1913 wiring and the weirdest newer extensions. In one bedroom there was a receptacle that didn't work. The ceiling light didn't work as well, the previous tenants had taken down the fixture and left us with five(!) black wires taped up with band aid sticking out of a hole in the ceiling. Plus there were about 4 light switches there.
So what the electrician found was this: 3 wires (switched phase, unswitched phase and neutral) came up to the light and 2 wires continued all across the ceiling and down the wall to the receptacle. As these bloody idiots had disconnected all wires at the fixture the receptacle didn't work. BTW, part of the wire to the receptacle were 0.5mm2 (something like 20AWG) zip cord.
