Last year I had a job teaching an officeful of secretaries to use their IBM XT.

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.