At our office we had a bunch of wireless keyboards and mice. Like what is shown below.
You'll notice the tiny USB plugin that is used to connect the keyboard and mouse to the compute. What is a little known fact is that many times you can actually have multiple keyboards plugged into the same computer and they work at the same time. So, now you know the science behind the prank... on to the story.
I had a manager that had a love to scare people. Sometimes he would hide under a desk to scare someone. I felt bad to try and reciprocate because he was an older man and not in the best health, so I was scared I would cause him to be put in the ER. So I decided to plug one of those bad boys into his computer when he wasn't around. I swung by his office to ask him a question (secretly I was probing what he was doing, so that I wouldn't ruin any sensitive information). About 5 minutes after I left, I pressed a button on the keyboard that would pop up the calculator shortcut.
I hear a mutter under his breath, "What the ****?"
I hear him click (presumably to close the calculator) and press it again. Again, and a little bit louder "What the ****"
At this time another manager whose name is AJ (who is also the IT guy) was walking by and inquired why there were 3 of us huddling around a keyboard giggling. When we told him, he got a huge smile on his face and asked for the keyboard. To which he pressed the calculator button about 30 times rapidly.
The manager who we were playing the prank on isn't very tech savvy almost screamed is fear "AJ!!!!!" The IT guy walked calmly in and inquired what the problem was. He looked over his shoulder and asked why he had so many calculators open? The manager tried to explain that it just did it. AJ said that that was impossible and why isn't it doing it now.
As soon as AJ left the office we pressed it a bunch of times again, hearing some very loud profanity and "AJ!!!! IT IS DOING IT AGAIN!!! HURRY!!!" At that point I came in with a big smile on my face and explained what we were doing.
He had a good laugh, then asked if we could plug it into another co-workers computer and give him the keyboard. We were thick as thieves at that point. We snuck into her cubicle and plugged the tiny USB in.
We waited for her to start writing an e-mail before the manager started pressing some keys. A little background to her, is she is a very religious person, and the manager typed "Amanda!" Which made Amanda stop typing and we hear a *gasp*. The manager continued: "This is God." Amanda then jumped up and started to hyper ventilate. We quickly explained the joke and she laughed as well.