Flummox your coworkers with an "Insert Coin" message on the office printer using the HP Printer Job Language (HPPJL) command set. The Hackszine blog describes how to customise the Ready prompt on HP printers' LED display with a few simple commands:
I haven't been able to test yet (no HP printer here), but if I'm reading the code right, you can actually do this right from the command line using telnet. Just telnet to port 9100 and type in the following:
\@PJL RDYMSG DISPLAY="MESSAGE HERE"
Sadly, I don't have an HP printer here to test this out either, but I'd love to see someone pull this off. If you do, let us know how it goes in the comments. (Oh yeah, and here's how to undo it when the boss hunts you down.)