I have a wicked streak, I freely admit this.
Usually, it is under control, but sometimes it breaks free of my iron grip, running madly through the corridors of my mind like a jackrabbit on Speed.
On one such occasion, I was walking through the typesetting floor of the newspaper that I used to work for, when a dreadful impulse came over me...
Now, this paper had long since gone over to electronic typesetting technology, but at the time, the typographers still had a strong union - strong enough to keep an iron grip of its own on the typesetting jobs. Ergo, all the text that was typeset on our nifty photosetters had to be manipulated, first, by a union member.
Crossing the typesetting floor, I chanced to see one of the more shameful examples of the axiomatic statement "I am a union member, so you can't fire me, regardless of my incompetency" bent before his monitor, hunt-and-peck typing away at the breakneck pace of 3 characters per minute.
Casting caution to the wind, I gave in to impulse. In my pocket, I happened to have a small but powerful rod-shaped magnet. I walked over to his station and bent over him, with a helpful-but-worried look.
Me: Say...there's something wrong with your monitor....
Me: Yeah, look!
(At this point, I waved my hand, holding the carefully-palmed magnet concealed within, across his monitor's face, producing a generous wave of displaced pixels on the screen)
Him: (starting) Oh my.
Me: Yeah. Looks like your monitor is broken.
Him: (appehensively) I didn't do anything.
Me: I know. (True, in several ways)
Him: What do I do now?
Me: (still holding hand near monitor) Well, you could get a new monitor. Or...
Me: ...or I could just take my hand away, like this. (the pixel dance stops)
Him: (wonderingly) It stopped.
Me: Yes. You see, I had a magnet in my hand. (turning hand over to show it)
Him: (not getting it at all) Did that fix it?
At this point, several of his cohorts had arrived to watch the show. Unfortunately, one of them was slightly smarter than the average, and understood the extent of the practical joke. Far from appreciating the joke, the typographers took it as a deliberate insult to their professional dignity (laugh, folks, that's meant to be funny) and for a while there, we almost had us a wildcat strike.
Honestly, some people can't take a joke.