In the scene where Bunny kills the one legged Vietnamese with the butt of his gun, watch the cigarette in Sgt. O'Neill's mouth. In a shot after the kid is dead, the cigarette in O'Neill's mouth is short. When it shows him again, the cigarette has mysteriously grown.
Director Oliver Stone claimed that the lead character of Chris was largely autobiographical and mirrored many of Stone's own experiences as a soldier in Vietnam.