Pranks always come at an expense, so you have to wiegh up the pros and cons of it. As long as it does not cost moment or hurt anyone they have their place. I am not a good prankster....
True dat. Harmless is the key. I don't think it's funny to make someone feel anxious, scared, in danger, or anything like that, then say ha ha just kidding. Causing confusion or ambiguity is acceptable - that's the essence of the prank - but not in a way that preys on someone's fears, which we all have.
Here's another one. I work in an office that consists of several large rooms arranged asymetrically. From the outside corner of one room you can see through a doorway in the opposite corner, a little ways down a corridor, through another doorway, between two walls, and all the way to the farthest corner of the farthest room, and distance of about 350 feet. Nerd that I am, I thought it would be fun to stretch a kite string along that full line of sight and tack it to the wall at each end. Just to make people think, "da heck is this all about?" Did this with another guy's help one weekend, but it was down when I got to the office the following Monday. Probably some maintenance man with no sense of humor.