For a Few Dollars More (1965)

Continuity mistake: Near the beginning when Lee Van Cleef looks at the the wanted poster on the ticket booth at Tucumcari (just after getting off the train), there are two extra zero's drawn onto the poster (as added by the villain). Van Cleef then takes this poster away with him, and slots it under the door of a hotel room on a later scene - notice that the additional zero's have since disappeared from the same poster. (00:06:30 - 00:09:20)

'Baby' Red Cavanaugh: I didn't hear what the bet was.
Monco: Your life.

Trivia: The 'whistling' heard in the background before the opening credits is that of the director Sergio Leone.