Last year I had a job teaching an officeful of secretaries to use their IBM XT. Well, for April Fools Day, I inserted a Pascal program at the beginning of the AUTOEXEC.BAT file (runs on startup). The program essentially said “Hello, Department of Defense Missile Network…” and gave instructions which led to “Missiles Launched”, and “congratulations, you have just launched World War III. Say goodbye to everything you love.” I slowed down the printing to match 300 baud, so it looked quite threatening. After the “say good-bye message”, I had it tell the user to hit RETURN, after which the program said APRIL FOOL and went on to the normal programs.
The results were interesting. The people who were comfortable with the computer loved it. The real computerphobe registered only that this wasn’t her database program, and (as usual) demanded key-by-key instruction, ignoring the prefectly good instructions on the screen. No-one really was startled, they didn’t have the background.