Factual error: The semi-auto handgun is not a real Bergmann 1911. It is a Walther P-38 that was modified to look like a 1911 Bergmann, but when Michael shows it, he puts the magazine in the grip rather than in front of the trigger, as a Bergmann would require.
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: 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]
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.
Revealing mistake: Whenever 'Dog' gets wet (like fording a river) his makeup runs and you can see that he is really 'Lassie'.
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: When Big Jake falls in the mud early in the movie (I think he's fighting one of his sons), his clothes are clean and dry immediately afterwards.
Continuity mistake: When Michael kills the sharpshooter in the bell-tower, the man is clearly dead when he falls out of the tower, but we can hear him scream on the way down to the ground.
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.