TL;DR: I created a rumor that our company was being bought out, which by sheer coincidence had implications at the CEO level.
Back in the mid-90s, the day before April Fools Day, I wrote a fake Wall Street Journal article about Toshiba buying up our company's stock, i.e. a hostile takeover. As a clue that it was fake, I made the Toshiba spokesman's name Igata Takashawa. I scribbled an illegible name on a printed copy and left it on a random manager's desk.
Within 2 hours my dept suddenly had an emergency all-hands mandatory meeting. The room was packed, and projected on the wall was my article. It was utter pandemonium in there, everyone talking at once. Because of April Fool's it was also "funny hat day". A few people had funny hats, but our manager had gone all out and was dressed like a pirate - black pants, big white shirt, red bandana and a giant gold plastic earring. He was British and strongly resembled actor Bob Hoskins - bulldog-like build, blustery attitude. When I walked in he was standing on a chair waving his arms trying to get everybody to shut up.
It seemed upper management was very disturbed by the article, and wanted whoever created it to come forward. They promised no recriminations (yeah right) and only wanted to know the reason for it. I kept my mouth shut.
A friend of mine, who worked near a bunch of managers and overheard their conversations, said their tone was like, "Why Toshiba?" and "Why now?" - as if the story might have a grain of truth.
It came out months later that by sheer coincidence our company had been working jointly with Hitachi on a top secret military project - a portable, battlefield ultrasound machine to detect plastic sharpnel, which didn't show up on x-rays. The engineers called it "the tricorder" because it resembled the one Spock used to carry. The boys at the top wondered if the article was a subtle message that the secret had leaked out, which could affect their ability to get future government contracts.
Anyway, I left that job within a few months but my friend stayed for 7 or 8 years, and he said every April Fools Day the "Toshiba incident" still came up in conversation.
Damn... Better than I got, but I still had fun.
On my way out from my mostly office job in the military:
Each desk had an overhead cabinet with cabinet doors that opened upward. One of my coworkers was constantly complaining and trying to fix a speaker wire that often fell out of his overhead drawer and hung down. So, first, I took a sheet of paper and wrote "Make it rain!" on one side. I then taped it very securely to the back of the cabinet door, with the writing facing down. On top of the paper, I placed hundreds of old pennies that had been collecting in a desk drawer for some reason. And then for a finishing touch, I made sure that annoying wire was hanging out so he'd want to open the cabinet immediately and fix it. With the inevitable result that he'd yank the door open, pulling the paper out with it, and once the pennies were past the edge of the cabinet, they'd fall and scatter everywhere, leaving the paper hanging there and reading "make it rain".
Boss's desk had a deck of playing cards on it that were never used. I took those and hid every single one of them in various places around the office. Underneath keyboards, between the pages of technical manuals, taped to the bottom of desk chairs, amidst the clutter in desk drawers, inside a coworker's lunchbox, in the freezer of the office fridge, behind computer monitors, under the wiper of the boss's car outside, on top of high cabinets, etc, etc, etc. Oh, and of course one sitting with the pennies from the previous prank, to fall down with them. Some of them would be easy to find and be noticed almost immediately, like one tacked to the office bulletin board, some would be nearly impossible to find and would probably only be noticed years later, if ever, like one tucked up into the ceiling tiles. But every single card in that deck found a new and innovative hiding spot.
As I was on the way out, I never got to see these come to fruition. I was gone by the time anyone noticed. But I did get a text message a day later that only said "You beautiful bastard". That was satisfying.