Noise on powerlines is annoying, very frequently present and sometimes dangerous.

Long ago there was this case of a factory that pressed desks out of steel sheets. Their main press (an absolute monster) had taken someone's arm off and they couldn't find the cause. It turned out that near the roof there was an air conditioning unit that that had a flaky relay in it that drew a gigantic spark on disconnecting, enough to upset the latch that controlled the safety interlock on the press, causing the press to move by itself.

It took quite a while to find it.