This was back in the early 1990s when we worked at a company making Macintosh utility software. My friend, another coder, liked to arrange his Finder windows so that they reflected the underlying folder hierarchy and were perfectly aligned. I mean pixel perfect.
So one day I wrote an extension to patch the OS on his machine to randomly nudge a window by just a single pixel. But only rarely, maybe just two or three times a day.
It had the wanted effect. Every once in a while, I'd hear him huff or mumble "what?" to himself. A couple of days later he asked the room, "Have any of you guys been using my machine? I know someone has. They messed up a few of my windows." Everybody professes innocence and manages not to laugh.
A few more days and he's at his wits' end. He finally says he's going to take the morning to wipe his machine and reinstall the OS.
At this point, we can't contain our mirth, and he catches a couple of us trying to hide our laughter. It's over.
I confess. He's pretty pissed and lets me have it. But he's a hacker, and I know deep down he respects it just a little.
After lunch, we're back at our desks. We're working. It's quiet. Then, I hear him chuckling. "Yeah, that was pretty good."
Edited to add: As penance, I ended up modifying my OS hack to help my friend perfectly align his Finder windows. All he had to do was hold down the control key when he opened a folder, and its window would be perfectly snapped to its parent. A bunch of us started using it. I ended up releasing it as freeware under the name WindowStacker: https://info-mac.org/viewtopic.php?t=14300