Percy Garris uses the phrase "Bingo" emphasize a point. "Beano" didn't reach North America until 1929. By the time it was renamed Bingo and made its way to South America it would have been well into the 1930s at least.
The locomotives on the trains are class K-28s, which were not produced until 1923, for exclusive use by the Denver and Rio Grande Western Railroad on their narrow gauge track in Colorado and New Mexico. (SOURCE: Model Railroader Cyclopedia Volume 1: Steam Locomotives, p. 81.)
When the engine of the special train blows its whistle to signal the posse, the sound is that of a single-note whistle. But the close-up zoom shows that it is a six-chime whistle, which would make a radically different, lower-pitched sound.
In the opening sequence when Sundance shoots the gun belt off the card player, the film was cut to make the quick draw appear faster. You can see Butch Cassidy's image jump across the screen in the background.
When Butch and Sundance blow up the safe on the train, their hats are blown off by the force of the explosion. A bare-headed Butch looks over as the second train arrives. The second train is shown for a moment and when the film cuts back to Butch, he suddenly has his hat on.
After the payroll guard Percy is shot he lands on the ground, Butch grabs the payroll bag from where his body is and as he pulls away a good deal of foliage lands right on top of Percys face. it sits for a while and then in the next shot as Butch and Sundance rush away, the scene shows Percy lying on the ground with absolutely no foliage anywhere near his face.
When Robert Redford is getting ready to be tested at shooting at the small object on the ground (missing), Paul Newman is standing to the side some yards away, but is not present in that position at all in the next shot when the gun is fired, but in the next shot on from that is then seen walking up towards Redford (amazed that he missed) from what must be just feet away given the short elapsed time in between.
In the final shootout in San Vicente, there are trees all around the town and one in the plaza. The real San Vicente, Bolivia is at 4800 meters altitude (over 15,000 ft.), so is well above the tree line.
When Sundance jumps onto the train near the beginning of the film, and makes his way towards the locomotive, it is obvious that a stunt double jumps into the coal car and Robert Redford emerges - not enough time elapses from the point at which the double disappears behind the coal until the point that Redford rises up from the coal at a different position.
When Etta lights the lamp on the table in her cabin, light fills the room, however the shadow of the fixture on the wall behind her is parallel to the floor, not angled upward as it would be if the light source was really the lamp on the table.
When Sundance enters Etta's house as Butch reads the newspaper, Katherine Ross can be seen in the background taking her apron off. Since she puts it back on immediately after walking through the door, it's likely Ross forgot to take it off between takes.
When Butch and Sundance are payroll guards for Percy and riding back up the mountain Percy is going on about becoming colorful. Just as he finishes saying he is colorful for the second time he is shot in the chest by a bandit from cover. If one looks at Percy's vest on the right side one can see a small patch. The patch is where the explosive is hidden that simulates the bullet as it strikes Percy.
During the climatic gun battle, just before Sundance returns to the room after covering Butch, he is shot and his reaction causes him to lose both of his guns. In the next shot, he has a gun in his left hand as he makes his way to the door.
After binding and gagging their sheriff friend, Butch Cassidy and Sundance Kid don't bother to take any rifles (which they had complained about not having) nor any fresh horses. They ride off double on the same tired mount they rode in on.
While the second train robbery is taking place, the Pinkertons arrive on the special train. The shot of the special train engine coming to a stop right in front of the camera was obviously filmed with the engine starting close to the camera and backing up, then the shot was run in reverse. Notice the steam is returning to the engine instead of coming out of it.