A coworker and I had some fun with a guy in the office who made the mistake of complaining that someone was stealing his Mountain Dew every afternoon. He's a good sport so we couldn't resist messing with him. Everyday we would do things to his soda.
He always brought in 2 cans of Dew that he taped together with his name on them. First day, we replaced the morning can with new one. Second day, we added 2. Third day, we drilled a hole in the bottom of a can and drained it, then replaced the afternoon can. This went on and on. We drained and refilled cans with Kool-aid, salt water and expanding foam.
The funniest part was that he'd run over to us everyday to tell us what the prankster had done that day and ask for ideas on who we though it might be. We gave him lots of ideas but they never seemed to work out.
He finally figured it out when both of us were on vacation and nothing happened. Somebody then spilled the beans and everyone had a good laugh. It's been 10 years and I still check under my desk everyday waiting for the reprisal.