Other mistake: After the brief fight between Barnes & Chris, when Barnes has Chris pinned, he cuts him below the left eye with a knife. It is a quick cut with a sharp knife, but instead of a thin bright red line we see instantly a blob of dark blood. This is plain sloppy.
Other mistake: In the scene where the Heads are smoking marijuana in their clubhouse, Sgt. Elias cocks his shotgun and tells Pvt. Taylor to put his mouth to the barrel. Elias puts his mouth to the breach and blows smoke into Taylor's mouth. When the shotgun was cocked, it opened and closed the breech. Elias couldn't have blown smoke through a closed breech.