In the 1979 episode of Three's Company, the one where Chrissy goes to to hospital with a head injury, if you watch in the beginning, when Mr. Furley knocks on the door, the camera goes too high when it shows the whole room, and Janet walking to the door, you see 3 big square holes in the ceiling with wires up there...And when Jack and Janet walk into the bathroom after Chrissy falls, you see the ceiling again.