When Remy removes a rifle from the wagon to show his gang, the rifle looks as though he has just removed it from a saddle scabbard. Brand new rifles being stored and/ or transported would be coated in rifle grease and wrapped in some type of waxed paper to prevent rust.
When they are dumping bodies into hole for burial, one of the dead guys in the hole moves his arm so that the next body doesn't fall on it.
At approximately 35 minutes, during a shootout, the stuntman's mattress is clearly visible on the ground next to the building from where he will fall off the second story railing when shot.
When Travis is taking the townspeople across the river on the barge, a man picks up a cross pole to let the people off the barge and he almost hits Lee Van Cleef in the face. Lee Van Cleef dodges the pole just in time.