The Camera & The Photograph

Early in The Iron Giant, Hogarth is trying to take a photo of the Giant, but gets bored while waiting for the Giant to appear. While waiting, Hogarth is cleaning the lens of his camera only to accidentally take a picture of himself. Later, Mansley discovers the camera and develops the photo that Hogarth accidentally took of himself, revealing the Giant looking over Hogarth's shoulder, thus finally giving Mansley photographic evidence that the Giant exists. And when Hogarth tries to deny it during his interrogation, Mansley uses the developed photographs to catch him in a lie.

Example of:
Chekhov's Gag