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: Gunny Highway insisted that they all wear the same shirts as he himself was wearing. If they showed up in a different one, he made them go shirtless. The reasons for this were 1) to break through their rebellious attitudes and teach them to follow his orders, 2) to make them look and feel unified, and 3) to develop resourcefulness and adaptability in them.
Chanteuse66
To also display esprit de corp to other units gaining them respect from those units as well as command leadership.