When your network admin doesnt realize you are a developer and get paid to fix problems. Tries to change your background to Justin(e) Bieber with group policies and you overwrite them in the registry. Now to plot revenge...

Any Linux machines in your network?
If so, have some fun with crontab and selfdeletings scripts.
A script loads, so you can make it rm itself, then make it reboot another server, then remove the crontab entry.
I guess you get the picture.

Takes a long time to find ghosts in machines ;)

Then again, this probably isn't "fun" anymore.

@forkbomber your name fits the comment