A few years ago I was playing a Russian mobster in a movie and there was a giant ground loop getting into both the audio feed (boom mic) and into the video (rolling bars).
We were loosing light, and although it's normal for lead actors to come back to overdub their lines, it is normally never done for a one line speaking part, as I had.
Everyone was in a panic so I stepped out of line and suggested to start unplugging things until we lost the ground loop.
It turns out it was the DAT (Digital Audio Tape) recorder for the boom; a very necessary part of the production.
We used a "cheater plug" on the DAT power cord that took a 3 prong grounded and turned it into a 2 prong ungrounded plug. This broke the ground loop and we finished the shot.
Now the same generator, lights, DAT, mixer and camera set up had been used together 100 times before, but on this occasion the DAT deck was somehow finding another path to ground and mucking up everything.
Another time a friend had installed a zillion dollar home theater set up and had a 5 channel dedicated amp to run 5 speakers. About a month into using it, just the front left speaker started humming loudly with 60hz hum.
He was blaming the amp manufacturer and swearing up a storm.
After swapping inputs I found that it would only hum when all 5 inputs were in use. Unplugging any of them killed the hum in the left speaker.
Lifting the Comcast feed cable from the rack instantly removed the hum, so we used an isolation transformer between Comcast and the rest of the equipment and all was well.
Somehow in the last month, Comcast had given itself a different ground potential and the symptoms showed themselves in just one channel of the amp.
Strange stuff these ground loops...