Character mistake: When Moss is arguing with the border guard at the Eagle Pass international bridge, he claims that he is a veteran of the "12th Infantry Battalion." There has never been such a thing as the 12th Infantry Battalion in either the Army or the Marines. Rather, they are based on a structure of 3-4 battalions per numbered regiment (i.e., 1st Battalion, 3rd Infantry Regiment/2nd Battalion, 5th Marine Regiment, etc). The film takes this seriously, as the guard, a veteran himself, buys Moss' story.
No Country For Old Men (2007)
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Directed by: Joel Coen, Ethan Coen
Starring: Woody Harrelson, Tommy Lee Jones, Josh Brolin, Javier Bardem
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Suggested correction: He might have meant 12th Infantry Regiment. From Wikipedia: "Three 12th Infantry battalions deployed to South Vietnam with the 4th Division from August through October 1966."
If he said "battalion" but meant "regiment", then it's still a valid mistake for saying it wrong and being believed.
Regiments have not existed as functional units in the US Army since shortly after Korea; they are simply historic names associated with various battalions. Marine battalions are not numbered higher than 4 in any regiment, and in any case do not carry an explicit designation of "infantry."
Llewelyn Moss: I'm going out.
Carla Jean Moss: Going where?
Llewelyn Moss: There's something I forgot to do, but I'll be back.
Carla Jean Moss: And what are you going to do?
Llewelyn Moss: I'm fixin' to do something dumber than hell, but I'm going anyways.
More trivia for No Country For Old Men
Question: The movie does not have a soundtrack at all, yet during the end credits, music is heard. What is the name of that music?
Answer: "Blood Trail (end titles)" By Carter Burwell, the Coen's frequent composer. The Film does not have a soundtrack, but you can listen to it in Carter Burwell's website: http://www.carterburwell.com/projects/NCFOM.html.