I got my old Warrant Officer one time pretty good. He had just gotten a new optical mouse for his computer that had the memory stick port on it and a few other fancy little gizmos. He was real excited about it since he's one of those tech geeks that always has to have to most up to date electronics.
So a couple of days after he got it, I went over to his desk after he had left for the day and took a small piece of paper and taped it over the optical pick up on the bottom of the mouse.
Next morning he comes in and his mouse isn't working, so I'm watching him from across the office, crawling under the desk checking cables, using the keyboard commands to go into the hardware setup to check drivers, back under the desk to plug in the old mouse, confusion now because the old mouse works just fine, back under the desk to plug in the new one, now he pissed because it still doesn't work. Then I see him pick up the phone and call the IT hotline. He's on the phone with them for 10 minutes and he's getting ready to try and reload the drivers, when I FINALLY see him pick the thing up and turn it over. As soon as he does he looks right over at me with a look that could that could kill because he knew I did it. I had wrote a little note on the paper, "Hahaha Gotcha!!!"

Seemed like a good time to grab a cup of coffee and go out for a smoke. The IT guys thought it was great.