Continuity mistake: After Hartman punches Joker, the scene shifts to Joker's point of view with Hartman pointing his finger directly in Joker's face while berating him. After the scene changes to a distant shot, Hartman is pointing behind Joker's head.
Character mistake: Hartman calls the "beds" bunks. Marines always refer to them as a rack.
Continuity mistake: When Hartman is messing with Snowball, Joker quotes John Wayne. Hartman seeks out who said something. He grabs Cowboy by the blouse and pulls it upwards forcefully while questioning if he made the joke. Then Joker admits to making the joke. Hartman moves over to Joker, but you can see Cowboy's blouse is now bunched up at the top and no longer properly tucked in. After Hartman reprimands Joker, he moves back to Cowboy. Cowboy's blouse looks as if was never touched by Hartman. (00:03:40 - 00:04:40)
Continuity mistake: When the squad is first ambushed they take cover around the corner of a building. Around the corner the window is boarded up. The next shot the window is unblocked and wide open and it enables the sniper to shoot Cowboy.
Factual error: When the Marines are in Vietnam, most of them have the M-69 flak jacket. This body armor was first issued in 1969, while the rest of the film takes place in early 1968. In addition, the vest would've been largely issued to the Army, not the Marines. Their vest would've been the M-55, which is different in shape and design.
Continuity mistake: Just before the jelly doughnut scene, as Hartman is walking up the aisle, he is in front of Joker's locker, about to reach Pyle's locker. A moment later he comes to Joker's locker, before he gets to Pyle's.
Factual error: Most of the Marines before and during the Hue City offensive, are wearing black leather combat boots. By 1968, these would've been mostly if not completely replaced by the jungle boot for Marines and soldiers in Vietnam.
Continuity mistake: When Hartman slaps Joker, Joker's head is turned sideways by the slap. A moment later, he is looking straight down.
Continuity mistake: During the scene where the Joker moves into the building to kill the sniper, the sniper sees him and starts shooting at the pillar he is behind. During this shot, large chunks of the pillar fall away, but in the next shot, when the sniper gets shot, the pillar is intact.
Factual error: The flashlight that Pvt. Joker is using on his last night of basic training is a "Mag Light" that wasn't invented until approx. 6 years later. And the "yield" traffic sign the platoon is jogging past is of the style that didn't come into existence until approx. 1973-74, (i.e. red and white). In the 1960's the sign would have been yellow with black letters.
Character mistake: When Sgt Hartman is giving the lesson about Charles Whitman, he says that he killed 12 people - in reality he killed 17.
Other mistake: In the first PT run if you look at their boots you see they have their trousers bloused, this doesn't happen in boot camp till week 3 or 4. In the next scene they are at drill and no bloused trousers.
Continuity mistake: When Hartman is showing the doughnut to the Privates, it changes looks depending on the shot: smooth and sugar coated, or wrinkled with barely any sugar.
Other mistake: Private Pyle seems to be just as large in the bathroom scene with Hartman as he is in the Jelly Donut scene. In reality, it's just the same actor, but in the film's plot it implies he went through 12-22 weeks of boot camp under Hartman without ever losing any weight, even though Hartman was specifically trying to get him to.
Continuity mistake: The shape of the beam of light coming from Joker's torch, after he enters the bathroom where Pyle kills himself, is different from the shape of the beam before he enters.
Answer: It was Gustav Hasford's idea. It happened in the original book that the story is based on, "The Short Timers."