Oh man, so many from which to choose.
#1: Back when hard drives were just being offered (only by third parties) for IBM PC's - yes, this was back in the early 80's - a coworker was relentlessly bragging to everyone about how awesome his system was going to be working and how fast it would compile his then-favorite language, Pascal. None of the rest of us were worthy, etc. It was finally too much for a room full of Engineers... a very dangerous situation in which to put oneself!
I recruited a couple of guys to work the sting. For the first one, I wrote a telephone script that had him impersonate an employee of the company shipping him the hard drive system. He got an outside line and called back through the switchboard so that our target's phone would give its distinctive "incoming outside call" ring. Partner #1 flawlessly convinced him that he worked for the drive subsystem company and then proceeded to tell him that all was well, his drive would be shipping soon, and that the only glitch was they'd discovered a minor incompatibility with just one application: Microsoft's Pascal compiler.

I was watching the victim from across the room and his excitement when he first answered the call from his "vendor" turned to complete dead silence when the bomb was dropped. Then accomplice #1 went for the killshot, and told him that not to worry, THEY had a solution: Their own Pascal compiler which was 100% compatible with his new drive system and would only cost an extra $1K.

The victim hurredly ended the call. Accomplice #2's desk was right next to his, and while the victim did his best to hush the whole thing up it was quite natural that his neighbor would have heard the whole thing so he hammered him until he admitted what was going on. Naturally a crowd had gathered at this point, and once he told us that his precious drive system wasn't compatible with his existing Pascal compiler I blurted out "Man, your money's GONE". Everyone else snickered and honestly we pretty much destroyed this guy... he knew he's overbuilt his position and then to have the air let out of his balloon so publicly bordered on cruelty. But it didn't actually cross the line, so we enjoyed every minute of it!
To let him in on the ruse, we set the script for the call on his desk figuring that once he read that he'd know he'd been set up. Instead, he thanked his neighbor (accomplice #2) for the transcript of the call! He was so bamboozled that it didn't occur to him there was no way he could have known what was actually said on the other end of the call. Finally we explained the whole thing to him and after a sheepish moment of self reflection, he openly admitted he deserved it. He took it well and toned down his attitude from then on.
#2: I was working at a super small startup, just a handful of men and women, in cheap rental office space. A couple of women had the ladies' room all to themselves, while all of us guys shared the one men's room (which was a single toilet+sink affair). The exhaust fan was deafening, such that when you were inside you couldn't hear anything and so you'd mentally drift off with your own thoughts.
One of the technicians disappeared into the bathroom and settled in. The doors opened out, so we carefully and quietly removed the hinge pins from the door. We heard the toilet flush, the door handle rattled, and then the door simply fell forward from its top and crashed onto the floor in front of the bathroom. The guy was thunderstruck, frozen in time, as his brain tried to resolve what had just happened to him. Never mind that literally everyone on the other side of the door was laughing to the point of tears....
#3: Same technician, same bathroom. He'd been in there a while, and someone pointed it out to the rest of us. He'd been in there plenty long enough to get "mentally complacent". So we all gathered around the door and on a signal, we all pounded our fists repeated on the door for a few seconds. It must have been like a string of firecrackers going off right next to his head. There was a moment of silence, then he shouted "Thanks, it helped!"

#4: Another technician, at a different company, assigned to keep our PC's running. I resented this because I didn't like people tweaking my (then-DOS) systems. But he insisted he needed to swap out the power supply on my perfectly working tower. OK, if you insist....

He left the tower in place under my work table, managed to get the cover off, and left the power on while he very slowly and very carefully reached toward the motherboard with DMM probes. He was being just a bit
too careful, if you know what I mean, indicating that he was a bit nervous about whatever he thought he was doing.
He got one probe in place. Then, just as he touched the second probe to whatever point he was seeking - I clapped my hands once, as loudly as possible, and made a "BZZZZZZZZZT" sound.
The effect was astounding. The entire table lifted off the ground as his head jerked up and whacked the bottom of it. He dropped both probes within the case (which thankfully didn't touch anything important). Then he collapsed into a heap on the floor under the table.
I was on the floor with him, laughing so hard I could not stand up. He composed himself for a moment, and then admitted "OK, I had that coming".