I came across a nifty prank to change the LCD readout on HP printers. The best script for this that I found was at IronGeek.com.
I used the script at CERN in the summer of 2010. It caught someone's attention. The American Psycho reference was suggested by Arun Luthra, so props to him.
I had a few floating around Buildings 40 and 54, as shown below.
I really liked this one. Burton DeWilde was just down the hall. Unfortunately, I don't think anyone bothered him.
A friend of mine, Ellie, found this one in the wild. It was requested by Arun.