When John brings in the rocking chair, bible and basket to Mary, there is a group of people sitting around listening to a man play a guitar. His hands do not even begin to match the music that you hear.
The sheriff is playing chess when his deputy comes in with Tom Elder's wanted poster. The sheriff's opponent, who is supposed to be the better player, moves one of his pawns backwards two spaces. Pawns can't move backwards.
John insults Bud for using the non-existent word "clumb" instead of the proper "climbed" in an effort to demonstrate why Bud needs to go back to school. But later in the film, John himself uses the word "hung" in reference to a hanging when he should have said "hanged."
When the Elder boys are in town and decide to go to look at
their old ranch, Tom has a scruffy beard. When they ride to the ranch and have a run in with the deputy, Tom is now clean shaven. They all ride back into town. When they reach town, Tom has his scruffy beard again.
John Elder arrives in town for the first time and goes directly to the sheriff's office. As he reaches for the front door to enter his jacket is hung up behind the handle of his holstered pistol, but when he comes through the door his jacket is draped over the gun.
When he is seen watching the funeral, the position of John Elder's hands change between the long shot (hanging by his side) and the close shot (his left on his hip, his right on his gun). A rock visible behind him in the long shot disappears as well.
After Matt is killed, John takes a revolver from Matt's holster and pulls the one he has in his holster and begins wildly firing them both until they are empty. Only problem is, the revolver that John had was already empty, and Matt didn't have one to begin with. They only had the revolver that John pulled from the deputy driver's holster when he stood up to tell his boys to start shooting as they were stopped on the bridge, and John pushed him off the wagon. Even if the revolvers and rifles all shot the same bullets, John's revolver was never reloaded and it was the only one they should have had.
When Tom and Bud are shackled together, the shackles are placed on Tom's left leg and Bud's right leg, as seen when they try to walk away. In subsequent shots, the shackles are on Bud's left leg and Tom's right.
When John and Matt Elder are shackled together at the blacksmith's, the leg iron is bolted just below the top of John's boot. In the next scene after Matt's leg iron is fastened, we see that John Elder's leg iron is clearly wrapped around his pant leg instead of his boot.
The shackle chains are about 5-6 feet long, yet when Tom and Bud go after the guns in the wagon at the bridge, they crawl along one after the other with their feet at least 10-12 feet apart and no chain sounds are heard dragging.
When Tom and Bud dive back into the water at the bridge after retrieving some rifles and shells, Bud is only carrying a couple rifles and no shells, yet when they surface under the bridge, Bud's arms are full with rifles and boxes of shells.
When Billy and Ben are at the train station waiting, Ben has his gun on. Then when they are walking into town Ben doesn't have his gun on and they go into the office and he puts his gun on to go to the funeral.
At the beginning of the movie when the Elder boys are at Katie's house, Mary Gordon rides in on a buckboard. She exits the buckboard with a food basket. The next shot shows the basket with bread hanging outside the basket and a red and white table cloth draped over the food.
When Bud says he is going with John and that they'll be famous like the Daltons, John replies that the Daltons were dead having been hung. Three of the Dalton brothers, Gratton "Grat" Dalton, Robert Rennick "Bob" Dalton, and Emmett "Em" Dalton all participated in a famous attempted two bank robbery in Coffeeville, KS on Oct. 5, 1892. Grat and Bob, along with two gang members, were gunned down and killed by town's people. Emmett, although shot some 22 times, survived. He went to prison until 1907. He died in Hollywood, CA July 13, 1937. An older brother, Frank, a US Deputy Marshal, was killed in the line of duty on Nov. 27, 1887. The Dalton's criminal career lasted just over four years. The photo taken of the four killed in Coffeeville is one of the most famous photos showing the eventual end of those who flout the law.
At the bridge, when the deputy jumps to catch the gun thrown by John Elder, he is driven backwards as he is hit by a bullet. This is physically impossible as there is no more force on the receiving end of the bullet as there is to the shooter. If the deputy is driven back three feet, so would be the man who shot him.
When Morgan Hastings is conversing with his son at his ranch while test firing a rifle, he bends down to fire the rifle and places it on a pile of sand bags. The bolt of the rifle is in the open position throughout the shot and so can't be fired.
During the gunfight between John Elder and Morgan Hastings in the gun shop at the end of the movie John looks up from behind the counter and Morgan fires a round at him. You see a glow on the counter where the bullet strikes the counter before the bullet gets there. This is where the small charge is that simulates the bullet strike.
If you slow motion the sequence where John Elder (Wayne) draws his pistol on the sheriff in the graveyard, you can see that there are two guns, one in his holster, and another in his hand, done (not very well) in an effort to make him look fast on the draw.