When Marquez is explaining the inscription, the positions of the three people in relation to each other and the rocks changes between shots.
McEwen is bitten by the rattlesnake when he puts his arm into a bush to hide his saddle. The next shot shows a rattlesnake in open ground nowhere near any bushes.
The mail hack was supposed to be leaving at 5 am, but the shadows are from an overhead sun indicating that the scene was filmed around midday.