If a unit doesn't has a ground wire in it's power connector, it is possible that the chassis has a voltage that can be measured between that chassis and ground. Your pc power cable is disconnected, but maybe your pc monitor is connected, and that one has a ground wire. So, in that case, you have a current flow from your television to your xbox chassis, trough your body to the pc chassis, and trough the video cable to your monitor and GND.
Those voltages are dangerous when you do things like hotswapping.
For that reason they say you should connect the shield of your xbox to the case of your pc before you attempt to hotswap.
It's never a good idea to connect a PC without a proper ground wire either.
The pc chassis can carry a voltage, and when you connect something to that pc, the voltage can damage the port to which you connect that equipment. I even have an external USB harddisk that creates a little spark when I plug it's USB connector in the pc. The harddisk supply doesn't has a ground wire, so the chassis of the disk enclosure carries some voltage. The pc chassis is at ground potential.
regards.