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]
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.
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: 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 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.
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 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.
Continuity mistake: When John Wayne is opening the note from his estranged wife delivered to him by a rider, the note is folded with the lettering on the outside, but in close up, Duke unfolds the note to reveal the lettering on the inside.
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.
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.