Coffer is seen using a Model 1903/A3 bolt-action rifle on the rooftop during the bank shootout scene. The /A3 model is clearly distinguishable from its earlier predecessor, the Model of 1903, by the rear sight placement on the rifle. The Model of 1903 used tangent sight located on the barrel, in front of the receiver, whereas the newer, improved variant used a "peep" sight located on the receiver bridge nearer to the bolt handle. The /A3 variant was a World War Two (circa 1941) improvement on the older model of 1903. Coffer should have been seen using a Model 1903, instead of a 1903/A3.
At least twice in the movie, characters refer to Gen. Huerta as the president of Mexico. This would place the time period between early spring of 1913 and the summer of 1914. However, when Sikes is telling his colleagues about a "flying machine", Pike informs them that he heard the machines would be used in "the war", which would place the time period late 1914, after Huerta had been overthrown. Also, at least twice during the film a character refers to Pancho Villa indicating he was a major figure in the rebellion against Huerta. During the revolt Gen. Venustiano Carranza was the leading rebel; Villa was a minor figure, although he did ally his forces with those of Carranza (he also later led a rebellion against Carranza after Huerta was overthrown). Mentioning Villa's name may have been an attempt to place a well-known name before the audience to give them an historical context.
When Lyle and Tector are shooting at the wine casks, the slide on Lyle's gun is locked indicating the gun is empty. However, shots are still heard. In the 1995 re-release version this has been corrected. Only one shot is heard after the slide locks on Lyle's .45, and that shot comes from Tector's revolver.
After the hijacked train crashes into the railroad car holding the horses of the army unit assigned to guard the train, the sergeant in charge orders a corporal to ride to the garrison and report the train robbery. The corporal salutes with his left hand and responds, "Yes, Sir!" Soldiers salute with their right hand and sergeants are neither addressed as "Sir"--except in basic training--nor saluted. A raw recruit might make such mistakes in such a stressful situation, but not someone who has been in the army long enough to become a corporal.
When Mapache is standing on the railroad tracks directing the battle against the rebels attacking the town, a soldier standing next to him is shot in the chest and falls face down at his feet. However, in a following shot, from behind Mapache, the dead soldier's body is nowhere to be seen. A few minutes later when Mapache turns around to board the train that is withdrawing from the town, that soldier's body can be seen but another dead soldier who had been lying on his back directly behind Mapache was missing.
During the opening shootout, a bald bounty hunter wearing a dark orange shirt and brown vest is killed with a shotgun blast. At the end of the scene, he can be seen exiting the hotel with the other bounty hunters, alive and well.
In the opening scene, all shots of the bounty hunters on the rooftop show heavy storm clouds in the background, but all shots from the Bunch's/townspeople's POV show a clear, sunny day (including over the roof of the hotel).
When Sykes (Edmond O'Brien) is shot by the bounty hunters while bringing the horses to Pike and the boys, it's the right leg that's hit. However, when he shows up after the battle at Mapache's headquarters, it's his left leg that's bandaged.
In the opening scene, as well as later, Coffer is shooting an "06". The rifle depicted is a .30-06, but it is not the M1903 then in use by the U.S. Army. It is a M1903A3 which was not produced until World War Two.
In the scene in which Angel shoots his girlfriend, and the Wild Bunch is confronted by Gen. Mapache's men, Commander Mohr asks them about their weapons, and informs them that they are U.S. Army weapons and cannot be owned by civilians. However, the only U.S. Army weapons which they possess are M1911 Colt pistols, which in fact had been sold commercially since 1911, two years before this film takes place.
The American soldiers in the movie wear a pattern of shirt which buttons all the way up the front. During this period of time, U.S. Army shirts were pullovers and buttoned part way down the front. The shirt depicted in the film is similar, but was not adopted until later.
Early in the film, Harrigan threatens Deke Thorton by promising to send him back to Yuma if he doesn't catch Pike. In reality, the Yuma Territorial Prison had already shut down in 1909, roughly four years before the events of the movie, and had been converted to a high school.
The M1917 machine gun, out of place in a setting of 1913 as already stated, was a water cooled weapon. At no time during the firing of the machine gun is the water jacket hooked up to a source of cooling water.
In the shooting in the beginning, one of the bounty hunters is shot and falls off the roof. When he hits the ground, one can see the ground break in where the air bag is hidden to break his fall (in the director's cut).
When the gang is crossing the desert and fall down the sand dune, the sun is behind them, casting shadows in front of them. During the close-ups (ostensibly shot on a stage) the shadows fall behind the actors as if the sun was in front of them.