Issues with the mains - when building TV facilities I always go to great lengths to keep the technical and domestic mains separate - well, to be more precise I ensure there is a clean technical earth derived from the same point in all areas where signals flow.
I had a worried couple of hours yesterday with one room that had huge hum over all the signals - running cables around the corridor in case the wiremen had run signal cables a long way along a mains tray made no difference - measuring earth discepency to the 16A C-Form connector on the wall showed no standing earth current yet when everything was plugged up the monitors looked like they didn't know the tune (humming!). Eventually I found an earth-neutral reverse in the Olsen block and the problem was solved.

Now, this client have their own substation, so I was even more suprised there was so much potential difference between neutral and earth, but was glad to have found the fault none the less.