Other mistake: It would have taken two hundred $10,000 wrapped stacks of $100 bills to equal $2 million. There is no possible way that suitcase could hold two hundred of them.
Factual error: The satchel isn't large enough to hold $2 million. Maybe $1 million. There were six stacks showing on top, so $60,000 across the top layer. Each stack is nearly an inch deep, meaning that the satchel would have to have been about 30 inches deep, in other words 2 and 1/2 feet deep.
Continuity mistake: During the dialogue between Moss and Wells at the hospital, there is an ashtray on the bedside table next to him changing position in various shots.
Continuity mistake: Near the end of the movie in the car accident scene, when Anton steps out of the car completely injured, the door's window appears broken in a way and then in a different one in a next scene.
Factual error: When Josh Brolin is leaning on the cinder block fence at the motel towards the end of the film, behind him are some mountains and a tall building. The scene is set in El Paso, Texas but the mountains shown are the Sandia Mountains which are east of Albuquerque, New Mexico.
Factual error: The phone bill shows several calls from Area Code 210 (San Antonio). This Area Code did not exist until 1992, some 12 years after the movie is set.
Factual error: Carla Jean says she works at the Walmart in Sanderson, Texas. There is no Walmart there, nor has there ever been. The nearest one is 65 miles away in Fort Stockton.
Continuity mistake: During the gas station scene, Chigurh is continually eating from his bag of nuts. During the conversation between Chigurh and the attendant, as the camera cuts back and forth between them, Chigurh keeps eating a nut from behind, and then from the front, his hands are by his side and he eats a nut again. This occurs several times throughout the scene.
Answer: It was a different gun. He kills Carson with a shotgun, while the gun Moss picks up is a silenced SMG by the looks of it.