We just replaced all the wiring in an old house, and tested it by running an extension cord from next door. We inserted 2 wires into the cord and then into an outlet. We ran a jumper from one side of the breaker panel to the other so that the entire house would be powered, and then checked it all. All the lights came on and all the outlets work (the 110's anyway, couldn't check the 220's). Afterward we were unhooking the extension cord and discovered that somehow the house was being powered by just HALF of the cord. With only one of the two wires connecting the extension cord to the outlet, the whole house was powered. How is this possible, did we do something wrong?

The other half was probably powered by a 240 volt appliance, feeding back to the panel.