One of the reasons that these pranks are pulled, in industry, is to point out the problems MANAGEMENT ignores. If you have someone on your project, or even leading it, that is causing problems, MANAGEMENT NEVER notices, especially if it's another manager. Pulling the prank highlights the problem and forces MANAGEMENT to do something about it.
When I was in college my roommate p!ssed me off. He had an IBM XT (showing my age here!) so I changed his command prompt to read "error reading drive C: Abort, Retry, or Ignore?" Funny as h3ll watching him panic. I didn't wait to long to let him in on the joke but he never p!ssed me off again.
Yes I am smart enough never to pull that kind of stunt outside of college.
I like this story.
In the past at the University of Stuttgart we had a similar, yet trivial hack for a chinese postgraduate: his VMS login script ended 'logout'.
You might imagine his astonishment :)
He was not too fit (thus the hack), so one of his colleagues hat to Ctrl-C his login and remove the 'addon' feature.
I narrowly missed the teletype - my first computer system used an IBM electric typewriter to communicate with the world. Many of our test programs were poor and I wrote one to test for specific printer faults. The program opened by typing an invitation to type out the user instructions, with a warning to new users that they should do this. It also typed out 'Incompetent Cretin' if incorrect inputs were given. One of the worst engineers that I had the misfortune to work with attempted to use the program without reading the instructions and, of course, entered a faulty command. Unfortunately I had accidentaly left a loop in the program, which then typed out 'Incompetent Cretin' repeatedly. He was hear to hysterics, and deep in continuous stationery, by the time I came out to see what the problem was.