Best prank I played was accidentally taken to another level by a co-worker.
I was hired to program batch COBOL on a mainframe in a city office. It turns out that there was a way to write to the screen and get user input. This was new to me, so I practiced it. (For all you computer geeks, it wasn't CICS, but I don't remember what it was.)
So at one point, I created a map that looked like an exact copy of the screen. So when you ran the program, it looked like nothing had happened. "Oh, it didn't work." I could say if he was sitting there, and then walk back to my cubicle.
Whatever you typed next, was greeted with " Program not found".
Usually, the person would just think he'd typed the command incorrectly, and would type it again.
"Program not found" again. Now they get a little worried or puzzled and carefully and deliberately type in the program name one letter at a time and check it was right and then hit Enter.
After which they saw:
"Look, Bud, I said 'PROGRAM NOT FOUND'."
This was followed by a few taunts and finally this request:
"Say the magic word."
If you said the magic word, it let you out (see below). Otherwise, it looped into "I'm not doing anything until you say the magic word."
The first person I pulled this on then gave me a mean idea before he tried it on someone. I added, at the end,
"You didn't sound sincere." for an added giggle.
I also had it check for user id, so I could tailor "Bud" to the handful of user ids I knew at the time.
He tried it on the woman who had trained us, who had a great laugh at it. So she immediately went to her friend's computer and launched while he was in the computer room.
HOWEVER, he comes back with a bunch of people including a woman who updated the network and (I think) took care of security. It was some kind of group talk going on about all sorts of things, and the woman, Valerie, sits down at this guy's computer to look something up.
And the fun begins. Joyce (the woman who trained me) is quietly sitting in the background watching the fun). The rest of them are totally perplexed at the prompts and don't know what to do.
Martin, one of the system guys, hears the commotion and walks over. "Martin, help us!" they call him. He reads the screen, "Say the magic word".
"Martin, please help us" she responds, not actually understanding what she had just said.
Martin apparently didn't get it either as he was ready to take the entire system down to get rid of this virus. Did I mention that this was a city agency. A NYC government agency. One that prints out lots of checks and stuff. Not a good thing to do without a good reason.
Joyce finally stopped it when she thought it was about to spin out of control. And Martin was a tad worried about me because he thought I'd actually found a way to hack the system after only two months there never realizing that they weren't actually in the system at the time.
I put an app on our sysadmin’s computer that reflected her first command as >Format c:\ on our UNIX system.
It responded with >Confirm: format c:(Y/N)?
No matter what she typed, it reflected a “Y” response from her and began ticking off the “formatted” sectors.
I heard her scream clear on the other side of the building.
Everyone was asking why she said YES to that question. She was pleading “I said NO”. But the screen showed her YES response.