I've been caught out by "ground" not really being ground before; two different sides of a whole floor computer / comms centre had different potentials. So most of the power was hooked up to ground on one side of the building and a small number of non computer / comms gear was hooked up to ground on the other side (think lights etc that were too difficult to rewire), but the other gear used wall sockets that were a different colour to make sure people didn't plug computers or network gear into them. This was all discovered after a temporary 20V spike difference in the different grounds trashed an expensive router. The electricians suspected that because we used a largish number of microwave links on the building the copper bars that run down the building to ground were acting like antennas and picking up the energy. So we defaulted all the sensitive gear to the "ground" that was least problematic. It was cheaper than re-doing all the ground circuits in the building with RF shielding. The joys of old buildings.