A lesser-known feature of the HP Printer Job Language (HPPJL) command set is that you can change the ready message” from “READY” to whatever you like. Yaakov wrote a Perl script that will change the message for you, and he also came up with the clever “INSERT COIN” replacement. Very nice.

You can think up your own funny, confusing or scary messages. My personal favorite is “INSERT COIN” which fits perfectly on the small LCDs. You can even sit in sight of the printer and change the message while watching the reaction of your victim. Don’t be surprised, though, if a large fraction don’t even notice. I was quite surprised myself but, it appears, some people don’t look at what is in front of them.

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:

e%-12345X@PJL JOB

INSERT COIN: changing the “Ready Message” on HP printers – [via] Link
HP LaserJet Commands (PDF) – Link