I was comparing several amps, had them sitting of the floor. One was a big monster with a bare aluminum chassis, and another a big Bryston. Was playing something a bit loud through the Bryston when I shifted it just a bit - plastering both terminals against the aluminum chassis. Loud noises and sparks.
Waited half an hour and powered up the Bryston again - and it worked just fine.
It was the last time I tried arc welding with an amplifier, though.