Computer pranks .....
OK - here's one I've done on a few people who didn't follow standard security practice....
Using the ISPF editor under TSO there are a set of function keys that have a set of default shortcuts. One of them is set to "END" which is the command used to exit an edit, among other things. Needless to say, this was used by everybody multiple times a day. However, you can edit these shortcuts to whatever you want - and you can even chain commands.
My trick was this ....
If someone were to leave their terminal without locking it, I would change this particular shortcut from "END" to "TSO AF;END". This would execute a CLIST (command list - like a macro) called "AF" which I had set up - and then execute the END.
This CLIST checked the date and time - and if it wasn't the morning of a particular date, it would exit immediately. The delay this caused before the "END" command was actioned was short enough to be accepted as normal - so this mod could have been laying unnoticed for weeks or even months.
On the morning of that special day (which you'll soon work out, if you haven't already) ... it did a little bit more
... but to add a little randomness, it did a little extra test on the time - the seconds, to be precise - and would exit immediately if it fell outside the condition set. This meant the "problem" would not be able to be reproduced at will.
BUT ... having passed all the tests, a series of messages were progressively displayed on the screen. These messages were (almost) valid ones which would normally indicate a very worrying situation. I say 'almost' because I changed the 'action character' in the message number according to a specific pattern. When it had finished the display and all the messages were visible, the screen just sat there, waiting for a response.
The action character pattern I used was: A, P, R, I, L and F, O, O, L - with a blank line in between.
I told the boss of Tech Support what I had done. He just shook his head ... and let it ride.
The real hassle was waiting for the fun.