Once I plugged in a wireless mouse into their computer without them knowing. And a few times a day I would just jiggle the mouse. Just enough to hear them slamming down the mouse and muttering under their breath and I'd stop. This went on for several days. Sometimes I'd stop by to chat, and I would bring the mouse. When they we go to click on something I would just move the mouse just slightly so they couldn't actually hover over what they wanted to click. It was brilliant!

The former big boss of my oil plant works in the office with me. I noticed anytime I would mention an interesting storey he would immidiatly fact check it and point out how wrong I was. So, If I wanted to know something , rather than look it up myself, I would just throw out a claim. Me- xyz stock is $4.40 today Him- NO ITS NOT - its $2.17
Me - Thank you

Me- Chicago is a 8 hour drive from here. Him- NO ITS NOT - its 12 if you do Route A, and 10.5 if you go Route B etc.

My coworkers think I have a cat. I've named him Winston.
I've pleasantly avoided many after-work happy hours and other work events with "I have to get home and feed the cat".
I even have a photo of some random cat on my phone in case anyone inquires further.
I don't have a cat, of course. But I do chuckle to myself at the thought of Winston, My Imaginary Cat.

Grabbed a roll of stickers from the pharmacy that say "For rectal use only" and randomly attach them to pens, phones, staplers, the water cooler.
It's all good till our director comes in and loses his shit and can't help but laugh.

I sit beside a guy who is a good friend of mine and our desks are separated by a moveable divider. Since I moved beside him 2 weeks ago I've been moving the divider a centimetre towards him each day. We're at 13cm and he hasn't noticed yet.
I have to shift his monitors soon though so that will be interesting.

Everyday I would get into the locker room before him and place one penny in his right boot. This went on for 2 months. After about a week and a half I could see him getting frustrated. After a while it just became the norm for him to shake the penny out of his right boot everyday. He wasn't frustrated anymore, he was defeated, and just accepted it now. So, one day I decided to put it in his left boot. He came in shook his right boot out like usual, but nothing fell out. He looked so relieved, like a huge weight had been lifted. Then he put on his left boot on, and just f*ckin lost it. He slung that boot across the locker room with all his might, cussing and calling out whoever did this to him. I stopped f*cking with him after that. I plan on putting a penny in his boot once a year from now on just to remind him. Nobody knows it was me placing the penny and I plan on keeping it that way.

Made a new folder on his desktop called Russian Dwarf Porn and then took a screenshot. Set the screenshot as his desktop background.
For an engineer it took him a ridiculous amount of time before he realised why he couldn't delete the folder.

This is only to my bosses, but when I know I'm due for a raise/promotion and they tell me they don't have budget/I need to wait a while, I start wearing suits to work. Not everyday, but maybe once a week, maybe twice, skip a few, repeat.
Looks like you're going to interviews during lunch or after work.

For a while my favourite thing was to unplug their mouse & put a sticky under the mouse sensor.
Was great when people plugged it back it and it still didn't work...