Well!.....I will tell you a couple of pranks that I did in my younger years!
I run a N/C milling machine here at work, and many of the guys will leave their maching cutting parts while they go to the bathroom. Well....I figured out how to black out the controller's screen so when they come back, the machine is working like crazy, but they cannot see any of the information. I simply slipped a black peice of paper between the monitor and the glass cabinet their located in. The looks on their faces was so funny, and to see them pushing buttons trying to get the screen to come back up had me rolling on the floor.
We also use a thick ink called Persain Blue to mark our parts with. It is easily hid, by putting it on any of the black switches on the machines. A clueless operator will soon find themselves covered with it, if their not paying attention.
aaaahhhhh!.....those were the days! Today a person would get fired for doing these typs of pranks.