Marshal Sharp alternates between wearing his badge (it is clearly displayed) and not wearing the badge.
While the outlaws ambush the posse as they cross the river. Graff grabs the Marshall and throws him into the water then shoots him in the head. In the next shot the posse and Graff are completely dry.
At the beginning or the movie, we see a screen banner telling us the story's setting is New Mexico and takes place in 1873. Many of the characters use a Colt Single Action Army Revolver, some of which show significant wear. This pistol was introduced in 1873. It is unlikely that such a large number of the characters in this movie would possess a weapon that had just been released, especially with noticeable wear and customization (nickel plating, pearl handle plates, etc.).
In one scene, a closeup of Sharp with Graff in the rain, snaps on Sharp's coat are clearly visible. Snaps were first patented in Germany in 1885 yet this movie is set in 1873.
The song that Graff repeatedly whistles, and is sung by the men accompanying him, is Unreconstructed Rebel aka Good Ol' Rebel. The song was not written until 1914, many years after the setting of the movie.
In the same raining scene above-mentioned with the snaps, the "Colt 45" held by Sharp is actually an inexpensive (but decent gun) Sauer and Sohn's replica that came out in the 1960s. There is no firing pin on the hammer.
When they reach the Rio Grande, they are heading south into Mexico, but the river is flowing left to right. When approaching the river from Texas, the flow is always right to left.
As the outlaws head "east" (after much discussion and emphasis that they are going east instead of south) their shadows appear on their right as they ride, indicating that the sun must be shining from the north.