Find your favorite HP laserjet printer in the office with one of those LCD displays that says "READY". I think this one only works on HP4 or 5s and up. Find the IP address it is attached to the network with and telnet to that address, port 9100, and send the following string:
←%-12345X@PJL RDYMSG DISPLAY="LOW COOLANT"
The message that normally reads "READY" should change to your new, rather confusing error. Great fun if you write a script that changes it randomly.
Someone even <a href="http://www.ancientpond.com/ljshen.html">wrote a utility</a> to do it for you if you pipe it to the network name.