I'm sure you recall the "floating ground" that was common in stuff made in the 50's and 60's. That was the first attempt to make household electronics safer. If one side of the line is connected to the chassis and you touched the chassis of two devices together - one plugged in one way and the other plugged in opposite - the two chassis would be at opposite potentials and sparks fly. So with floating grounds, that doesn't happen but it still means the chassis would "float" to about 70 volts.

These are why most guys use an isolation transformer on the old stuff - Trying to keep from letting the smoke out of the scope. Remember those 1-to-1 isolation transformers?