This is an old trick, but always fun. It’s easy to change the status message on a networked HP printer; just telnet to the printer’s IP address on port 9100, and type the following:
@PJL RDYMSG DISPLAY="YOUR MESSAGE HERE"
Press enter, and you’re done; the new status message will stick around until you change it back, or someone restarts the printer.

If you want to put some effort into it, maybe make an agent to change the message at a random interval, you can also write an application to accomplish the same thing. See here for an example in C#.
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Are you able to give a bit more detail about changing the status message of a HP printer. I am not that familiar with computers but would like to give it a go. How do I telnet to the printers IP address on port 9100? thanks
Adrian
As the sole IT guy, I FINALLY managed to weasel some time off out of my boss. So why did I think it would be incredibly funny to change the HP printer status message to “VIRUS LOADED - READY” the night before I leave?