We used to play pranks on each other such as logging into the NeWS server on a colleagues machine and manually setting a small rotation in the transformation matrix for a terminal window that someone was typing in....
NeWS had an interactive PostScript shell and almost no security so this kind of mucking about was trivial...
For april fools' day, on my first job as a troublesome ~16-year-old administering our university lab's firewall (academia was a different land when it comes to trust), I set up some automatic network-wide substring replacement filters for incoming HTTP responses to replace `<body` with something like `<body style="transform: rotate(0.1deg);"`. This was in the time before HTTPS was ubiquitous, so it worked on most websites.
Unfortunately, it broke some pages that lab users needed for school. I later learned one colleague wasn't able to complete a homework assignment because of my prank.
Ah, good times.... we used to do similar things across the HP Apollo workstations at the place where a friend of mine worked (and I unofficially "borrowed" computing facilities for a project of my own -- though I did also contribute a substantial speed optimization to their main product, so nobody seemed to mind).
NeWS had an interactive PostScript shell and almost no security so this kind of mucking about was trivial...