Last week at work, we proved that ground loops with an oscilloscope can be a bad thing. Here is a description of what happened, but the pictures mostly speak for themselves. As I understand it (I was not present) the issue occurred when a setup involving 277 volts was present. Our oscilloscope was not plugged into earth ground through the third prong, which was supposed to help look at AC signals. Unfortunately, the 'scope was plugged into the PC via the serial port (to wave capture) and then the serial ground provided at path for potential disaster. Basically, all the grounds in the circuit should be at the same point (voltage), ours were not.This picture is from the serial port on the back of the 'scope. Notice anything odd?
Same board, different angle. In this picture you can almost see the lifted traces on the board as well. That trace goes from one of the pins on the DB9 to the chip. The yellow cap. next to the chip isn't looking great either.
As it turns out, since the loop was through the PC, it was not spared either. Shockingly similar failure to the 'scope. Lucky for all of us, the hard drive was spared any damage and a duplicate PC was located to replace this lost one. The 'scope, however, could not be replaced as easily.