UPS plugged into itself
We received two RTC UPS'es (3kVa) which we need to set up with new equipment that was going to be shipped out to a remote site.
One UPS beeped to indicate that it was unplugged from the power, then a click and then silence as equipment stopped working.
Some doofus plugged the UPS back into itself, causing it to trip.
A quick Gibbs slap to the back of the head of the offender, a >clickety<>click< and an unplugging later everything was fine.