In the final scene at the sheriff's office, Mitchum has his feet up on the table top, and the boot soles are clean. But after Wayne walks out, there is suddenly a spot on a boot sole. (This is visible in the video)
El Dorado is actually part of a remake trilogy by the same director, Howard Hawks. The first was 'Rio Bravo' (1959); 'El Dorado' was next in 1967; and the final variation was 'Rio Lobo' in 1970. El Dorado is the only humorous (and by many critics acclaimed as the best) version of the three.