A while ago, I had a co-worker who was also a good friend. He was always so fastidious about locking his computer when he was away from his desk because he didn't want anyone messing with it.
I must admit, I would have been MUCH more offended about this if I wasn't so busy trying to figure out ways to get onto his computer to mess with it.
One week, he was gone on vacation, and I thought, "Now's my chance!"
However, no matter what I tried, I couldn't figure out a way to access his computer to do anything crazy. The best I could do was change all the settings on his monitor so everything looked purple.
Then ... success. Rather than tell you how I did it, I will first tell you the results.
When he came back to work, he had to print several resumes for job applicants. He printed the first one, and found that it had a large "Sample" watermark in the background.
At first, he thought it was some new, creative way people are using to make their resumes stand out (i.e. this is a "sample of myself). Then the next one printed with the same watermark.
No matter what he printed, each page came out with a "Sample" watermark firmly in place.
He spent about 20 minutes digging through his computer settings, trying to figure out how I had done this to his printer. Finally he gave up and called me into his office.
I came into his office giggling like a little kid. I immediately opened his printer tray and removed all the papers I had previously placed there with the watermark already printed.
Isn't that brilliant? I didn't have to touch his settings, and I still stumped him. If he had printed enough documents, the final page had a watermark of a rubber chicken.
I'm completely juvenile, but I can't help it. Now I'm just waiting for memories to fade in my office so I can do it again ...