Continuity mistake: In the scene where John Wayne picks up the red box of money he lifts it onto his back and walks to an open door. As he goes out the door the camera moves to outside and we see John Wayne come out of the building. Now the red box is completely on his shoulder, rather than his back.
Revealing mistake: Early in the movie when the maid is opening the drapes to let in the morning sun, the sun lights the room a half a second before the drapes are actually opened.
Continuity mistake: When Jake's younger son Michael decides to show daddy his expertise with a rifle, he picks his target as "that tall tree over there, the edge of the small broken branch", and the Duke says, "Son, that's over 400 yards!" Michael says, "Nearer 500." There's no way the Duke could have seen that branch at 500 yards, because they clearly showed earlier in the movie that his eyesight was really bad. [He's long-sighted, not short sighted]
Factual error: The scene where Jon Wayne's Son is shooting the deer at 600 yards, he swings his rifle about 20-25 inches in about 2 seconds - by my calculations the deer must have been travelling around 250 mph.
Continuity mistake: When the Duke is in the shower at the barber shop, and he shoots his shotgun through the shower door to kill the bad guy, it blows the door apart, but the bad guy doesn't even have a little blood on the front of his body after he is killed by the mighty blast.
Continuity mistake: As James is being hassled in the bar, one of the bad guys goes through the window into the hotel room. Sam shoots him and he falls backwards through the balcony railing. The next time the railing is shown, it's put back together.
Revealing mistake: When "Dog" is chewing on one of the men on horseback's arms, after he lets go, the blood on the guy's arm couldn't look any more phoney.
Continuity mistake: There are four men in the third car when crossing the bridge. When going through the stream there are now five men, and when the ambush happens only three jump out, leaving one dead man to go over the cliff. (00:40:30)
Continuity mistake: When the gang of bad guys ride to the ranch there are nine of them. When they leave, and throughout the rest of the movie there are only seven, despite only one being shot off his horse at the ranch. (00:15:15)