When Jake and the newly-formed posse run into Jake's old gang, he punches one of the members in the mouth, breaking one of his front teeth. Later in the movie this gang member has all his teeth.
When Jake is riding towards his house, the scene is a long shot but it can be seen very obviously that the bracelet is missing from his arm. In the following shot the bracelet is suddenly back on his arm.
When Jake is riding his horse to rescue Ella who has been picked up by an alien ship, his horse changes three times. He starts on a light bay with a star. Then he is riding a bay without a star. Finally, when he is leaping from the horse onto the ship, he is riding yet a third horse, a very dark bay.
After escaping the mine, both Jake and Woodrow are covered in dirt.
In the next take, they are clean.
The three cowboys Lonergan meets after first waking up appear very dirty and sweaty, and all of their equipment is old and used. The cowboy hat he later takes from one of them is obviously new, as it doesn't appear dirty and doesn't have a sweat salt stain where it would rest on one's head.
In the opening scene, Jake grabs a knife with one hand and the shotgun with the other from the father, who is readying to shoot him. He pivots on one leg, stabbing the son's leg with one hand, while holding onto the gun with the other, which is still being held by the father. The son is at least six feet from Jake, and another two feet from the father, so unless the father cooperated fully with Jakes's hostile actions, or Jake jumped (which there is no sign of), Jake's arm length would have to be at least eight feet for him to complete the stabbing.
As is usual for Hollywood all women have shaved legs and armpits which was never practiced in this time period.
Sheriff John Taggart identifies himself as Emmett Taggart's maternal grandfather ("Your ma's buried here, and you know I don't want to leave my little girl."), and the boy's father is supposedly still alive. If so, then the Sheriff and Emmett should have different last names. It is possible that the boy was very young when his mother died, and his grandfather raised him with the Taggart name, whether or not he legally adopted the boy.
When Woodrow and the rest of the men on the trail of the aliens arrive on horseback at the upturned boat, it is raining heavily. However, no rainwater appears to be dripping off the brims of the men's hats.
The boot Jake takes from the dead cowboy has a suspiciously clean sole.
The three cowboys Lonergan meets after first waking up appear very dirty and sweaty, and all of their equipment is old and used. The cowboy hat he later takes from one of them is obviously new, as it doesn't appear dirty and doesn't have a sweat salt stain where it would rest on ones head.
When dynamite is about to be lit they grab a matchbox made of cardboard, up until well into the 20th century (around the 1940s) matchboxes were made of wood.
Dollarhyde gives a knife to young Emmett that he claims he obtained as a boy. The knife is obviously stainless steel. Stainless steel wasn't invented until 1904 and didn't become available to the public until decades after that.
When Jake Lonergan asked Sheriff Taggart what he was being charged with, one of his charges is "hijacking," a word first coined in the 1920s.
Bronc (the Mexican in the gang) proposes to travel to Puerto Vallarta. In 1873 Puerto Vallarta was called Las Peñas. The name changed in 1918.
When Jake is looking up through the hole in the roof of his house, there are two crossed boards held together by a screw with a recessed head. The square or Phillips headed screws were not invented until the early 1900s.
At the beginning, Lonergan is using a rock to try to get the device off his wrist. Each time he hits it, he gives a loud grunt. But each grunt sounds the same, whether his mouth is open, closed, or has his tongue slightly coming through his lips. The grunts must have been added in post-production.
At the first scene when the father gets off his horse, a piece of bright blue equipment (possibly paper) with some tape on it is seen right at the saddle.