When I was working at a four-screen cinema as projectionist, I purposely spliced all four of our "No Smoking" trailers upside down, to see if the staff was paying attention (as well as my relief projectionist). It took 4 full weeks before anyone noticed..one of our regular patrons mentioned it to me in the lobby.
Needless to say I had some words with both the staff and my relief operator..they are supposed to check the screens every show when the "No Smoking" comes on, as it is the last thing before the feature.
My other fav was while I was in the Air Force..one pilot and I were engaged in a practical joke war (his fav was to detach the practice bombs from the racks and complain that the bomb was loose and "just fell off"). Well at a party the pilots hosted, I pulled the center table out from under him during a game of "carrier landing." (They run across the room and belly flop onto a series of table, trying to land as short as possible.)
The next day the poor guy could hardly move, and after his flight he was very pale from the pain. We called an official truce on the spot.