Factual error: When 8-ball gets shot, in the shot which shows the platoon ducking near a wall with Cowboy, Joker and Animal Mother in the front, there are two weapon related errors. Cowboy's M-16 has a nickel (silver colored) firing assembly and Joker's M-16 has an A2 type pistol grip. The A2 types were introduced somewhere around the 80s, and nickel plated assemblies are rarely used (and were invented after Vietnam, too). The nickel assembly is visible at the beginning of the shot, and you can see the A2 grip when Joker is changing magazines (while Cowboy shouts cease fire).
Factual error: In 1968 the USMC were still using the Sikorsky UH34/S58 helicopter (powered by a piston engine) but the helicopter seen in the film is the British version of the same aircraft, the Westland Wessex - gas turbine powered, it has a quite different nose shape from the original American version. (00:47:47)
Factual error: In the sniper scene where Animal Mother disobeys Cowboy to go help 8-Ball and Doc, and Animal Mother peeks around the corner of a building to try and see the sniper and the sniper shoots at him, narrowly missing his face, the impact of the sniper's bullet on the wall and the sound of the gunshot occur a split second before the muzzle-flash in the distant window from where the sniper is shooting from. This would imply that both bullet and sound have travelled faster than light. (01:37:15)
Factual error: Joker would not have been allowed to go through basic training and out into the field with wire-rimmed glasses. He would have been issued plastic/cellulose-framed Regulation Prescription Glasses (AKA BCGs or "Birth Control Glasses") like Private Cowboy's.
Factual error: When Pvt. Pyle commits suicide, he shoots himself through the skull at point-blank range with the tile wall of the bathroom only inches from the back of his head. The wall becomes splattered with blood and brain tissue but no hole in the wall is visible. Both the shot through Hartmann and the one through Pyle would have punched through the bathroom walls and quite possibly caused injury or death to anyone on the other side. (00:44:09)
Factual error: The film is set in and around 1968. In one scene, a copy of Ursula LeGuin's novel "The Word for World is Forest" is seen at Joker's bedside. This novel was not published until 1976.
Factual error: At one point Joker is doing chin ups, with Hartman motivating him. Joker does three chin-ups, but not a single one of them was done properly. You're supposed to lock your elbows all the way out on the way down, and your chin needs to clear the plane of the bar. Joker never locks his arms out, barely gets over the bar. In reality Hartman would have been all over him about this, drill instructors are notoriously strict about proper technique.
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.
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.
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.
Answer: It was Gustav Hasford's idea. It happened in the original book that the story is based on, "The Short Timers."
Captain Defenestrator