Heck... all you gotta do is print a bunch of pages with nothing but a header saying something offensive about whomever is in charge...
then load the printer back up with paper with one of those pages about every fifth page or so.
It's staggering how long it will take for people to figure out what's happening. could take days. several meetings will erupt about it.
Someday I hope to pull off the mother of all April fool's jokes. Not sure yet just what it might be. One really great idea was to use Active Directory to push out a new desktop wallpaper that looks like the Mac OS X desktop and a .reg file to hide the windows start button and task bar.
I once kiosk-mode'd Internet Explorer on the bothers' computer. It took them ages to figure out what was wrong.
I'd love to replace the Linux desktops with one that says "Windows R00lz" just to see how long it would take the *nix dudes to hunt me down.