Another one, which does tend to fall into the "mean" category.
When I was in high school in the drama club, I was on the set construction crew. We frequently spent the week there, with enough change of clothes and we would clean up in the boy's locker room. Our parents would bring in massive amounts of food and drink to keep us fortified.
There was one guy who did very little work but ate much more than his share. One day we decided that payback time had arrived. A few of us made a "contribution" from our overloaded bladders to an empty container of fruit juice. We added just enough fruit juice to make the color look right.
Later that evening, we found out our greedly little compatriot had made a raid on the refrigerator, and had consumed half of the container of the ringer "fruit juice" He could not understand why the lot of us were rolling on the floor in uncontrollable laughter. When we regained composure, one of us asked him if he thought the fruit juice was a bit "off".
He replied "Now that you mention it, it DID taste a little flat".
Several of us who were in on the joke told him what we did when we saw him at the ten year reunion. I never saw a face turn so green.