Here's a funny video by Kendrick. To hear hear him ask if anyone knows how to put a tube in is hilarious. He also pushed the input jack ground away from ground so the amp would hum. And then hammed it up. I've seen techs do this with rookies and even some more harder to detect stunts like tie a 1 ohm carbon resistor across the outputs of a power amp. When the unsuspecting tech powers up the amp, they get smoke. (but don't do this unless you know for sure the protection circuitry is working correctly in the power amp)
I'm sure there's more, let me know if there is.