Take a thumb tack and poke a small hole in the cup and place the cup in the holder at the water cooler. You can do one cup in a stack, every 10th cup or every single cup. The choice is up to you.
When you place the cups in the dispenser, have the holes pointing out towards the person when they take a cup. This will increase chances they do not see the hole and that the hole is on the right side of the cup to be useful for our prank. Besides, most people are not going to inspect the cup for holes.
Now sit back and watch as you create an office version of the wet work shirt contest.