Insert Coin
Yaakov writes –

This little perl program allows you to set the “Ready Message” on HP printers to whatever you’d like. (if you want to run it under Windows, and don’t know how, read this.) I wrote it after coming across the command in an HPPJL (HP Printer Job Language) reference manual I was reading for some reason that I now forget.

Well, of course I couldn’t ignore such an opportunity, and it turns out to be a lot of fun. 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.

Change the “Ready” message on HP printers to say “Insert coin” – Link.