Put #define true (rand()%1000 != 0) in some obscure C or C++ header file on their system/project. This makes true evaluate to false one in a thousand times, and will make them spend hours trying to figure out why things like infinite loops, aren't quite infinite.

Other languages should also allow you to do things like this, if not messing with constants, messing with standard library functions.

Our kids swapped the position of our bowls and plates in the cabinets, and our knives, forks, and spoons in the drawers. They definitely won.

I closed the tool palette in the illustration app of a coworker and replaced it with a same-size image of same.

I've heard it said that the key to a benign prank is to creates a mixture of momentary alarm and confusion, followed by relief.

A few years ago, I put a sign on my boss' office door that said that his office was off limits for the week, as the building facilities team was going to be proactively placing humane termite traps in his office for two weeks as a preventative measures to lure any termites in and then release them at a nearby forest. He found it amusing.

Tape over the mouse laser is always a good one

This one is a little convoluted, but I just watched this prank video where they fed someone their own cooking in a restaurant. It may be the most wholesome prank I've ever seen.

Hide small plastic babies around their house

True story. This thing happened last week.

Person A: I want to eat something.

Person B: Do you want some of these (B opens a box of boring cookies and shows the contents to A)

A: No!

B: How about these (opens another box of bland cookies)

A: No!

B: (grabs a third box, realizes it’s empty, but opens it carefully not to show anyone what’s inside) Ooh, I’m not showing this to you. (Puts the box away.)

A: I wanna see! (Tries to grab the box)

B: No, it’s mine! (Pushes A further away)

After some futile struggle B let A have the box. A opens it up and realizes it’s empty. We all had a good laugh.

Print out a little dickbutt sticker, then put it over their mouse sensor.

I saw a video once of a prank where two girls go up to a stranger asking them to take a picture of them across the street. The stranger takes the photo and when the two girls start crossing over to the other side, a car passes by and the two girls are replaced with two old men wearing the same shirts. Even the Polaroid photo shows the old men. It's genuinely funny and completely harmless. It just makes the person getting pranked go, "Wait, what the fuck???"

I do hope the people were told that they were pranked though and that they don't live the rest of their lives thinking they're insane 😂

ETA: also this video where the guy puts on the collar and leash of his blind husband's service dog when he asks him to go get his watch. (He immediately reveals himself when the blind husband gets back to pick up his service dog's leash again and the husband thinks it's hysterical).

If they leave their phone unattended take funny selfies and then quickly return their phone to exactly where they left it.

I put post-it notes all over my partner's workspace while they were out of town.

Replace the eggs in their fridge with convincing fakes that don't quite seem right.

I made a "spray grenade" to my friend once. Attached a Ty-wrap to a bottle of Febreeze (trigger in) and throw it in his room while he was there. He was pretty quick to seek scissors and cut it but I can imagine it worse without his quick thinking and a worse scent.

One of the best pranks I saw involved a house sitter, and some musical Christmas socks. Before going away, they removed the music module from an old pair of Christmas socks that played "jingle bells". They then put it between the mattress and the box frame of the bed a relative would be using, while house-sitting. When they turned over in the night, they would set it off. They had jingle bells stuck in their head for a week, but didn't know why.

On another note, a work colleague is the master of ear worm deployment. He has the knack for humming catchy tunes just right. Just a few bars, quite enough you don't notice. You suddenly find you have a tune stuck in your head. You know what he's done when you realise that half a dozen others also keep murmuring the same tune (it's stuck in their heads too). It's remarkably self reinforcing. It amuses him greatly , while being fun and non-harmful.

Find a c header file on a program they're working on.

#define main main

Dad jokes

• Short sheet their bed
• If they leave their computer unattended/unlocked:
  • Screenshot it, make that their wallpaper, then hide icons & taskbar
  • (unsure if this still works) Alt + Ctrl + Arrow keys to rotate Windows desktop
• Get a musical birthday card, then hide it (opened) somewhere in their room/office (best under a piece of furniture/couch cushion, etc) so all they can hear is the muffled music
• Remove spring from tip of clicker ballpoint pen (ink cartridge won't stay out/in)