Got my brother in law pretty good at work one day.
We were developing a new digital amplifier for car-audio. He was the accountant of our fledgeling company, but would come to the back of the shop nearly every day and make it his business to "test" the amplifiers. We really couldn't say anything because we were all partners in the venture. At any rate, he wound up destroying half of our prototype amplifiers by running them under excessive loads. Each of these amps was hand built by me, and took me about two days each to build. The engineer was also pissed at him, so we decided to "cure" him of his evil ways.
I took a soda can and filled it halfway to the top with Triple F blackpowder rifle gunpowder. I then took a thin piece of copper wire and shorted out an extension cord with it, and placed the wire inside of the can. I stuck the can against the wall under the test bench which consisted of a 4x8 sheet of plywood on top of 2 55 gallon steel drums.
As usual, my brother in law sauntered to the back of the shop, sat in a rolling chair, and commenced destroying the amplifiers in puffs of smoke. Another employee who was completely unaware of the prank came back to use the bathroom. I said "Hey John, would you mind plugging that extension cord in for me?" He says "sure", squats down, and plugs the thing in.
BEHOLD THE FIRE AND SMOKE COMETH! Flames about 8 feet high start shooting up from the concrete wall behind the test bench, along with an ungodly amount of smoke. My brother in law, thinking he was the cause of the conflagration starts pulling every wire he can find to stop the hell he thinks he created. This lasted about two seconds until he completely disappeared in the cloud of smoke. His will to stop the fire succombed to his survival instincts and he decided to abandon ship. He emerged from the smoke peddling backwards in that rolling chair as fast as he could, arms and legs flailing.
The guy who plugged the extension cord in, then made the profound statement: "Hey, something blew up".
After he saw us laughing, he realized he'd been had and starting chasing me around the shop yelling explatives. To this day I can't bring that incident up without him suddenly deciding to kill me.
But he didn't mess around in the back of the shop anymore, and the company has become a great success.