Continuity mistake: When Highway walks into the bar for the first time, his old friend the barmaid pulls him a pint and the beer is level with the glass handle. When he drinks from it the beer is past the top of the handle.
Factual error: During training and the invasion of Grenada scenes they show the Marines wearing PASGT Kevelar helmets. Marines did not wear PASGT helmets until 1984. The invasion of Grenada was in 1983.
Factual error: If Highway has been in the Marine Corps for 24 years, as Major Devin pointed out at the 9:45 mark, that would put Highway's entering the Marines at approximately 1959 (the beginning of the movie shows July 1983 as a timeline reference). But the Battle of Heartbreak Ridge took place in Sept-Oct of 1951. Moreover, Sargent Major Choozhoo was regaling "Stitch" Jones about the exploits in Vietnam that earned Highway the Congressional Medal of Honor. Two separate wars. (00:09:45 - 01:19:50)
Chosen answer: I can personally attest, when you see someone in dress uniform, the first thing you look at is their rank. No one really pays much attention to the ribbons, and not many people know what more than a handful of them mean. When you add the fact that Highway put the fear of Gunny into them at the first meeting and that none of them are all that bright, it could easily be overlooked. One thing that is wrong, is as an MOH recipient, Gunny Highway is entitled to a salute from anyone short of the President. Granted he is not the type to point out his awards either.
Grumpy Scot