Over the years I have done the following:
1. Screen capture and make as background - same as above
2. Scotch tape on RJ45 connector
3. Loopback plug on the patch panel in the switch closet
4. An old DOS one - edit the config.sys file to point at a file called ATUOEXEC.BAT instead of AUTOEXEC.BAT. Then create a System, Hidden, Read-Only file ATUOEXEC.bat with all kinds of funky programs. Sit back and watch reboot after reboot. Had the nice affect of keeping the simple CRC check of CONFIG.SYS in tact as the swapping did not impact the result.
5. When left unlocked - sending out "I love You Guys" to everyone in the department.
6. Changing peoples home pages to questionable content.
7. Forwarding phones into a sequence around the office. - Back when the old PBX system would actually ring the phones once before forwarding. Then just call the first number in the group.
8. Setting peoples Automatic Replies on their email to - "Why are you bothering me with this stuff? Just so you know it has been deleted."
9. Changing the sound profile.
Usual don't get IT pranks because people are too worried about what I might do in return. Although I have been victim of the home page change.
Unintentional prank, DOS, was when I was configuring an old UNIX laptop with NIS so when the guys deployed to the exercise they could use their network credentials and have access to intelligence. I forgot to tell it not to respond to requests from other systems and to only authenticate for itself. I had removed all the rest of the users except those needing access to the laptop. Well the next day when everyone in that wing of the building turned on their workstations they saw the laptop as the first SOA to respond. No one except in our little group could login. I had a very interesting discussion with the Colonel and the Security guys that morning.