Many of the actors playing Marines wear their hair too long. Additionaly, many sport sideburns. According to Marine Corps grooming regulations, sideburns will not extend below the top of the orifice of the ear.
In Nichiren Shoshu Buddhism, prayer beads include two strands which hang on the left hand and three strands on the right while chanting with palms pressed together. When Donna is seen in profile chanting for Meadows, her prayer beads appear opposite from the correct arrangement with three strands on the left hand.
Badass & Mule take off their SP bands and white utility belts before brawling with the Marines in Penn Station NYC, and afterwards run off without them. Much later when leaving the Boston Hotel, they're wearing them again.
About 50 minutes into the movie, the trio are walking by Gate 4 at the train station. One of the Marines they are about to fight is walking ahead of them. Moments later, the same Marine and several others are walking behind them.
The MAA Master Chief is not wearing a Master-at-Arms rating badge, he is wearing a Boatswain mate rating badge.
The Master At Arms rating was disestablished in 1921, but was officially re-established on 1 August 1973. Therefore, as the story takes place, a Master Chief Boatswain's Mate being assigned the collateral duty of MAA is entirely accurate.