What are some harmless ways to prank someone?

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.
- Your kiddos are brilliant lol
- This is the correct answer.

I closed the tool palette in the illustration app of a coworker and replaced it with a same-size image of same.
- I go for a variation of this if a coworker leaves their computer unlocked when they leave for lunch or the day.
  Screenshot their whole desktop, hide all icons and the toolbar. Set the background to the screenshot. Boom a computer that looks exactly the same as before but doesn't work.
  Quick, non permanent, takes a few minutes to work out, lesson learned to not leave your machine unlocked.
  
  Putting a piece of tape over their mouse eye is good also.
  
  That's a classic. We did it for a while. We moved on to all out warfare.
  We had a script that backed up the current desktop exactly the way it was then changed everything about it. It would change all system sounds to the original My Little Pony theme, change the background to a Friendship is Magic wallpaper, change the mouse pointer to a tiny pony with trailing sparkles, and remove edit rights to those files from the account using it. To get things back to normal you had to take ownership of the affected files.
  One guy couldn't figure it out and disconnected his internal speaker because someone had set up some policy deep in the system where he lost sound settings. I don't know how that worked. Apparently neither did he.
  Another guy worked remotely the next day so his computer just kept playing the song while he was gone.
  The guy who created it didn't get a normal birthday cake. The boss got him a My Little Pony birthday cake that year.
  What a lot of folks didn't realize and never asked about was that there was a reversal script in the user folder.

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
- On the computer thread, changing the keys on their keyboard is also fun. Works better if they are a touch typist as well, as they may go several hours/days before noticing, until they have to look for that one special key. :)
  
  Numpad * and / are good for that, I touch type but always have to look at those.
- If we're talking computers, screenshot their desktop, set it as background, move taskbar to top, auto hide it, right-click desktop and choose hide icons.
  Advanced mode: also set screen flip to 180 degrees and flip the background so it looks right side up.
  
  I did the screen flip thing to a coworker once. And then he just took the monitor and turned it upside down. Didn't see that coming.
- Plug in a wireless keyboard/mouse and do things, like move the mouse, or write "chicken" every so often?
- I added this to another similar comment:
  Follow it up by placing a tiny bit of clear soft plastic in with the laser. It doesn't stop the mouse from working, but if you were you draw a straight line the cursor wobbles slightly.

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.
- This British grandma is adorable lol
- So heart warming!
- That's the best thing I've ever seen!

Hide small plastic babies around their house
- .... are you my brother?

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.
- Follow it up by placing a tiny bit of clear soft plastic in with the laser. It doesn't stop the mouse from working, but if you were you draw a straight line the cursor wobbles slightly.
- Shouldn't it be one of those "OK" symbols. You know, the "Made you look" game where you create an OK symbol with your hand below your waist.
  
  That one?
- Jesus Christ... Dickbutt

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).
- Here is an alternative Piped link(s):
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  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source, check me out at GitHub.
- Basically everything Just for Laughs does is gold. You can spend hours watching their videos.

If they leave their phone unattended take funny selfies and then quickly return their phone to exactly where they left it.
- oh, I'm going to do this one to my partner
  
  I have another in mind but haven't tried it. If they leave their phone unattended, take a picture of their phone sitting there. Then later on text them the photo and say "You left your phone (wherever)" You will get them to view the photo of their phone on their phone and if you're lucky they will go looking for it when they already have it.
- If they use android, you can open the camera without unlocking the phone. Just double tap the power button, and it pops up. It's intended for rapid camera use, but great for pranking purposes. Apple has the equivalent, though I can't remember the button combo, off the top of my head.
  
  Wow. Been using Android since like version 2 but never knew you could double press the power button to turn the camera on. If screen is off have to press once to turn screen on first.
  IIRC on iOS there is a camera icon on the lock screen.

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

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.

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

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
- Does defining in a “loop” work in C? Like
  
  #define main A #define A B #define B C … #define Z main

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)

Dad jokes

Empty a can of corned beef hash into the toilet and squirt ketchup over it and then don't flush.

Send them a head of white cabbage every week (can use an online grocery service.
If you have access to their dwelling, put all their bedroom furnitures in the living room

I once hid in a box for, like, an hour so I could leap out of it at my brother.

Lift the toilet seat ➡️ Put cling wrap over the toilet bowl ➡️ Lower the toilet seat back down ➡️ Enjoy
- Alternatively, put it across a doorway. I did this to some devious children (6 and 12). They eventually got me back.
- idk if that's harmless
- That's not harmless that's cruel 😭

Detonate a nuclear bomb
- Just a prank bro

Throw a sack of flour in their face
- I don't think that's harmless...

Burn down their 10yo Minecraft base.

Bait t-shirt?
Edit: Bait Airpods then.
- Here is an alternative Piped link(s): https://piped.video/watch?v=WST_7c-mVA0
  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source, check me out at GitHub.

Piss in pizza box, freeze it, then leave it in front of someone's door if they're throwing loud parties.
Liquid @ss is astonishingly effective, can get it cheap on Amazon
Buy something that makes noise periodically, hide it. They'll go nuts trying to find it.
Record satanic rituals on a voice recorder. Tie string around it, open window quietly, lower voice recorder to hover above downstairs neighbor window. Set it to play. Can freak religious people out.
- None of those really seem harmless.
- They have tiny cricket noise gens.

Just don't. Pranks are a form of bullying.
- Lol are we in high school?
  
  Do you think that bulling ceases to exist when you graduate? That unwanted violence against others is imaginary, or that you need to endure it in silence because you're an adult?
  How many people have you victimized just because you can? Give your answer, I'd venture the number is non-zero.

Pranking is just what assholes call being an asshole.
Don't be an asshole.
- Believe it or not, friends can be mischievous with one another in good faith
- Pranks can 100% be harmless and fun, but if you're ever questioning whether or not a prank is harmless don't do it.
- A good prank is defined by whether the "victim" is laughing about it as well, at the end. The catch is, this is defined entirely by the victim. There is no arguing about it. If they aren't smiling, you screwed up.
  I've done, and been victim to some good pranks over the years. I've also dealt with the bullying version. There is a big difference.