Lister really didn’t like Craig much, but that was no excuse.
Lister wrote a background job which hacked Craig’s xmodmap. Every thirty seconds, the ‘g’ and ‘h’ keys would quietly swap positions. If you hit the ‘g’ key, and ‘h’ would appear on the screen, and vice versa. A second later, the keys would quietly swap back.
It didn’t steal his passwords, delete his files or lock him out. It just ever so slightly increased the number of typos he thought he made.
To my geeky mind, this remains the dirtiest prank I have ever heard.