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.)
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.
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 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 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.
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.