I played a prank on one of the women in the Accounting department last summer when I was an intern at the law firm. It was harmless and fun. I replaced her chair with an identical-looking reject I found at the office that happened to have a broken hydraulic lift resulting in the chair's sinking to the bottom of its height adjustment as soon as it was sat on. I also switched everything around on her desk so that anything that was on the right side was in its corresponding place on the left side. As a result of this move, I gained a reputation as something of a prankster in the office. This was not my intention, though I am not necessarily surprised.
For Christmas, one of the gifts my managing attorney gave me was rather peculiar. She gave me a prank kit of 12 pranks. I can't decide if she's condoning my prankish behavior or challenging me to apply it, perhaps on her. At either rate, she already blew my cover by giving me a box of pranks in front of the entire firm. Of course our firm's gift exchange was done with the entire firm present (there are only about 15 of us) so everyone knows that I am now in possession of a prank kit.
At either rate, I might use the boxed pranks as diversions for more sinister pranks. No one is safe.