After Calvera and his men are driven from the village the first time, there is a sequence in which three of his men start taking potshots at the villagers from the trees. One of these shots strikes Chico's hat and knocks it off his head. He even sticks a finger through the hole after retrieving it, however in a scene just a couple minutes later, Petra (the Mexican girl) is talking with Chico. We can see there is no bullet hole in the front of his hat and at one point, he turns his head 180 degrees in order to look behind him and there is clearly no bullet hole in the back of his hat, either. It has simply disappeared.
The first man Calvera kills near the beginning of the movie has no wounds on his back after being shot and falling to the ground. When the villagers run to the body to look at the man, there are two wounds on his back.
When Chris and Vin begin driving the hearse up to Boot Hill, they pass the Belmar Hotel sign twice - once silently at the very start, and then again as they briefly discuss the towns they've come from a few moments later.
When Calvera and his gang first ride into town in the beginning of the movie, they are seen taking chickens and food. When they ride out of town, they do not have any of the loot with them.
After Britt throws the knife into the cowboy in the rail yard, two train engineers are seen leaning out of the engine's window observing the scene. In the next shot, one of the engineers has moved to the platform between the engine and the tender car.
Toward the end of the movie, when the 3 little Mexican boys are praying over Charles Bronson's grave, one of them makes the sign of the cross backward, as a Greek or Russian would do.
After the first gun battle between the 7 and the gang, Britt is seen emptying his revolver, presumably of spent casings, however he's 'shaking' out the rounds and not using the ejection rod. Any spent casings will have swollen during firing and will not fall freely from the cylinder, as he is seen doing.
In some prints, Natividad Vacío who plays Miguel, is listed as playing Tomas, while John A. Alonzo (billed as John Alonso) who plays Tomas, is listed as playing Miguel.
The whistle Bernardo gives to the little girl DOES have a fipple, but he turns it down when he blows into it. You can just see it as he finishes making the whistle.
The steam locomotive when Britt duel with the cowboy (knife x revolver) is completely cold. No smoke is seen in the chimney as there is no steam coming out of the safety valves.
Odd all this because the freight cars were loaded with cattle on board by the cowboys and the crew apparently waiting for departure orders. Steam locomotives are not switched on or off at any time. They need hours to be used.
At 31.30 until 31.33 minutes AND at 31.38 until 31.39 minutes, at the railroad station, and the cattle yards of the RIO GRANDE CATTLE CO, during a skill contest a man is killed by a thrown knife. Correct for the era you see a wooden telegraph pole, that incorrectly does not have telegraph wires, unfortunately incorrect for the era that wooden pole only has the three wires of a standard USA two phase 115 volt electricity power wires.
When Chris and Vin are returning from the cemetery on the hearse, they come to a stop and get off the hearse. As the camera pans slightly when they're getting off the hearse you can see a piece of wood used as a chock (looks like a 4"x4") placed in front of the right rear wheel of the hearse. There was no one around to place the chock under the wheel.
At 58:19 when two peasants drop rocks on the ground it is obvious that the "rocks" are neither real rocks nor heavy by the way they fall and roll on it.
Basically in all the death scenes for the main characters, they are able to speak their last words or die slowly with all the gunfire from the final big battle mysteriously disappearing until they die.
During the final shootout, a number of farmers are killed. However, once the gun battle is over, only the bodies of the dead bandits are seen on the ground.
Chico taunts the red "toro" like a Spanish bull fighter, but it isn't a male "bull" rather a female "cow" with horns.
When Chico arrives drunk at the cantina, he shoots toward Chris twice, but doesn't cock the gun between shots.
As Brad Dexter is dying, as he talks, suddenly all the gunfire disappears. When he dies, the gunfire starts up again.
Steve McQueen wears Levi's jeans with the famous red tab. These were introduced in the 1930's, decades after the setting of this movie.
The knife used by Britt appears to be a stiletto switchblade (automatic opening knife with locking mechanism). This kind of weapon was brought from Italy only in the '50s which is about 100 years later than the action of this movie.
Vin is telling a humorous story to Chris and the Old Man and mentions a fellow back home falling off a 10-story building. In the 1870s, the period in which this film is set, no 10-story building had yet been constructed. This goof may be due to script/casting changes. The original script called for the gunmen to be much older Civil War veterans with Spencer Tracy originally envisioned as playing Chris, which would have placed the intended setting in the 1890s. With the first 10-story building being the Home Insurance Building built in Chicago in 1885, Chris' statement would have been accurate. With the casting of younger men and moving the calendar back, updating this line was apparently overlooked.
Charles Bronson wears a denim shirt. While Denim trousers had been around for many years the first shirts were not manufactured until the 1920's.
Audio is out of sync when Bernardo spanks the village boy. An additional "slap" sound is heard shortly after he finishes spanking him.
The boom shadow can clearly be seen moving from right to left as Sotero turns while addressing his fellow villagers after Calvera's first visit near the opening of the movie.
When Bernardo pulls one of the village boys onto his lap to spank him for calling the adult villagers "cowards" the bed appears to collapse beneath him.
Steve McQueen is wearing a wedding ring throughout the movie.
Chris tells Harry Luck that the job pays a gold eagle plus room and board. But the payment offered was $20. An eagle was the $10 gold coin; the $20 gold coin was the "double eagle."
When talking about what food the villagers are providing the Seven, Charles Bronson's character-represented as being of hispanic ancestry-pronounces the word enchilada as though it were French, sounding like onchilada.
At the start of the last shootout, Vin draws and fires on the bandito to his left in the doorway, then fires two more shots directly ahead, yet there are 3 bad guys sitting at the wall. There should have been a fourth shot fired in that scene if he was being thorough.