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.
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]