There's a story that I once heard about one of my former coworkers that called the FA up front and said that they had a "lav contamination" light, and could she go back and just take a look at the lav and see if anything seemed abnormal.

Of course, the FA would go back and check, only to come back and say that the lav was ok... After they landed, the CA would go to the back, discreetly hiding a pair of dishwashing gloves, a water bottle, and some blue food coloring. He'd put on one glove, pop the lid on the bottle, color it blue (including making sure that some dye ran down the side and on the gloves), then walk back up to the front.

He explained that the airline next door had a "lav water quality analysis kit" and hand the other glove to the FA and ask her if she would go over and ask them to test the sample... Apparently it always made for great fun watching the reaction of the other airlines employees.