Coworker also never locked his PC.

Company has a profanity filter on the Exchange server.

So one day we went into Outlook on his PC and edited his signature file and added the word "shit" to it, but we changed the font to white so it was effectively invisible.

The bouncebacks from the Exchange server were in plaintext, not HTML/rich text so he could see the "shit" that got his message bounced, but couldn't figure out where it was coming from.

At my last job, I worked in the service department of a new car dealership.  Every now and then, usually around holidays like Memorial Day, Fourth of July, Labor Day, etc., we'd have a little cook out for everyone in the dealership.  Burgers and hot dogs and whatnot.  We'd have it in the service shop, clear out a bay, set up a few tables, line up the food and everyone chows down.

One guy who had been there forever had a prank diaper he kept in his toolbox, it was new and clean of course, but he smeared RTV sealer or something in it so it looks like what you'd think it looks like, a shitty diaper.

So a bunch of folks are in line fixing their places, sitting down to eat, the diaper is planted in a car in a bay next to where the tables are.  Our prankster is sitting there eating and he goes "Guys, what's that smell?  Y'all smell something?  Man, it's awful..." and he gets up and starts walking around sniffing, he sticks his head in this car and wretches, he's selling it hard, "Dude, it's in this car!"  All of us are in on the prank of course, all of us except poor little Paula, a sweet gal they hired in the cashier's office about a month before this gag.  He reaches under the seat and finds this diaper, and of course everyone else is selling it, like "Ewwww" and "fucking disgusting" and so on.  He looks at it, shrugs, and licks it.

Paula faints.  She was standing in line getting something to eat, right in front of this guy, and she turned green, looked like she's about to puke, her eyes roll up into the back of her head and she just drops like a sack of potatoes.  We're all scrambling to make sure she's okay.  They had to go smelling salts out of the first aid kit, it was a funny and well intended prank that went horribly wrong, that we couldn't do anymore after that.  She was a good sport about it and if we knew that's how she was gonna react, no way in hell would we have done this, not even would have thought about it.  The dude that pulled the prank, he's a good man, an honest, God-fearing family man, and he felt terrible about the whole thing, he/we were just trying to have a little fun.

Nowadays, with HR departments and men and women not knowing what the hell bathroom to use, the line is too fine IMO between harmless prank and getting PIP'd.  All it takes is someone taking it the wrong way and things can go sideways really fast.  Sucks though, having a little fun at work helps the day go by.

Placing 1/2 bolts under each leg of a desk and when it drops there is not much of a clue.

Limburger cheese in the mouthpiece on a phone is harmless enough I guess.

Placing a rubber glove just right under a forklift tire sounds like a .22, just sayin'.

Switching large & small gloves or putting left hand ones in the box, you can imagine the results.

And the paper towel machines had centerfolds coming out as it was cranked was always good for laughs.

I wish they never told me what they did as I got blamed for a lot of it.