Stalag 17 (1953)

In director Billy Wilder's black comedy:

• the scene in which suspected traitor Sergeant J.J. Sefton (William Holden) in a WWII POW camp received his first clue as to the real villain's identity while on his bunk and under the shadow of a naked hanging lightbulb (serving as a signal - with a knot or loop in the cord) - while the others marched around the barracks singing "When Johnny Comes Marching Home"