It reminds me of some of the tricks I used to play in my youth (well, my early career), back in the days of Windows 3.1.
- Change the screensaver to call up Calculator, or Notepad, every few seconds (I wonder if you can still do this)
- Change the mouse buttons, or the sensitivity on the X and Y axes — one really fast, one really slow
- That old favourite, putting a desktop full of icons on as a background image, so you couldn't click the icons (significant in Windows 3.1 when minimised programs sat as icons on the desktop)
- Swapping keyboards or monitors with computer next door
- Swapping individual keys on the keyboard over (didn't work for touch typists)
- Spoofing a frequently-used icon to look like it had sent an abusive message to all the network users (followed by another colleague tapping the victim on the shoulder and saying the boss wanted to see him in her office).
- Emails advising of an imminent audit of pirated software (this caused a rapid deletion of files before anybody could stop the victim).
Low-tech tricks worked too. The pisstake-de-resistance was one morning putting a notice on a late colleague's desk to advise him that he'd been moved to elsewhere in the building. (This was met with an accepting “Oh.” and shuffled departure.)
The scary thing is that all of the above tricks were tried on one person, and he fell for them every time.