When my roommate and I were working at NOAA during college, his boss was ready to play a joke on him. He had a new circuit built, an o-scope ready to test, and his back to the hallway. His boss walked up behind him with a tall, empty metal trash can that he was going to drop when the probe first touched the circuit. Unfortunately for my roommate, he had a hot AC line where there was supposed to be a low-voltage signal. The probe flashed and self-destructed, and his boss never got the chance to drop the can. An orignal ROFL followed.
Here's one some of you may recognize: In the old days before test equipment had data storage and file I/O, we used to do data collection with Polaroid cameras that nicely fit over the display of the scope or spectrum analyzer.
The Polaroid film needed to be coated to preserve the image, and each film pack came with a plastic tube containing a squeegee full of the coating goo. The cap on that plastic tube made a very tight seal.
The plastic tube, filled with dry ice, capped and casually dropped into the victim's lab coat pocket would eventually warm up and the CO2 pressure would blow the cap off with a loud popping noise :)
In high school we had an electronics program. The classes were taught in a detached "trailer" that had its own electrical service. The service was located in the back of the trailer; first one there got to walk in the dark to the back and flip on the breaker. There were desks in the middle facing the board, with electronics stations lining the sides. These held a scope, DVM and bench power supply. One day I decided it would be funny to wire small electrolytics (1uF) across all the power supplies backwards and turn the supply on with the main breaker off. Whoever got there first would turn the breaker on and before they got to the front of the room the caps would be exploding. Well, it was the teacher that got there first, and once everything settled down he was not happy. A "little" joke got out of hand when someone decided if a small cap was good, a bigger one would be better. They did the sme thing but with something like 4700uF. The unreal mess was not expected, and someone could have got very hurt (ok, I know, you could get hurt from little ones as well, but I was a kid). After that anyone doing it would be expelled. That was also the class were we decided wiring a sign transformer (15kV) to the handle on the door was funny as well...
When I was in school in the late 60's, We would have to build up circuits on a proto board, and hook up a bench supply.
Several times we concealed old-fashioned photographic flash bulbs in the protoboards, and wired them to the power. When the victim flipped on the power supply toggle switch to start work they were greeted with a huge flash and pop.
I also remember removing and reversing the cardboard sleeve on foil capacitors. The victim would then not know that they had wired the capacitor in revese polarity. When powered-up this would produce an startling explosion. Sometimes it would un-roll the capacitor.
As per previous comments, playing jokes like this with 3-phase mains power is neither funny nor clever. The flash from evaporating copper wire can cause serious injury. You can get just as good results without such dangerous practices.
This joke was quite a bit louder and much more exciting, but not very funny. I was working with an electrician and we were in a utilities closet, which was behind a janitors closet, all concrete walls and ceiling. What we did not know was that as a gag, I guess, somebody had dropped some pieces of #10 bare copper wire into the conduit that fed the 3-phase 480 volt power to the main breaker in the panel we were looking at. When the electrician pushed on one wire to get a better view of the terminals, the chunk of #10 wire fell down across all 3 phases, and instantly evaporated, with considerable flash and bang. The acoustics in the small volume made the sound even louder. We never did figure out just who or when the trap was set, which is very fortunate for whoever did it. It was not very funny.
Years later I worked with another electrician who thought that it was quite funny to tie all three phases of the feed to a large motor, downstream of the starter contactor, together with about #20 wire, and then have the engineer, (me), see if I could spot the trouble when they went to start the 75 HP motor. Those watching were dismayed when I was not amused.
Years ago I was working on the B1 bomber gyro platform and I was using a number of fans to cool the electronics box. One night another engineer brought a big blower to the empty bench next to mine and plugged it into the same outlet that ran the box. He had filled it with chad, little punches of paper from card stock used to program an old HP computer. The next morning, I turned the box on and instantly my world was filled with blown chad. I immediately shut it down and everyone thought it was pretty funny. It took me a half day to clean it up.
Then, I noticed that only a small part of the huge pile of chad actually blew out. Plotting revenge, early on Thursday afternoon I returned the blower to him, placing it on the empty bench next to his. He made some comment about any left over chad but I smiled and mentioned that he had no worries as it was all over at my bench. That brought about another round of good laughter.
I came in on Friday morning around 5 am. I plugged it into his bench and covered the power cord with some of the piles of papers on his desk. I also placed two "decoy" power cords next to the blower, thinking he might unplug them first.
True to form, at 8 am he turned on his bench and his world was filled with chad. What was even funnier, is that he didn't figure it out at first. Then when he did, he kept unplugging the wrong cords to shut down the blower. By the time he got the blower off, all of the chad was all over his desk. We both were called into the engineering manager's office and got a lecture on that one.
Once a colleague was repairing a measurement unit a customer nearly ruined. It took all morning and he was cursing a lot. He was really annoying.
When he was sure the repair was finished, he left for lunch.
In the mean time we placed a 0207 resistor underneath the unit, which was housed in a 19" 21 TE cabinet, wired it to a power supply well hidden on the next desk and adjusted the voltage. When our colleague returned, he continued - trying to bring the unit into service again.
You might imagine his reaction when he switched the unit on and - as the "auxiliary power supply" was activated at the same moment - a thin trail of smoke escaped from the overhauled unit. That was the time we defined new classes of parts: LET = Light Emitting Transistor, SER = Smoke Emitting Resistor, LER = LE Resistor (use a bit more power and you can remove the coating and watch the metal film glow).