Revealing mistake: When the Marines are jogging through the training camp complex, singing "I don't know but I've been told..." you can see the British road junction markings on the tarmac underneath them. There is little evidence to suggest that there was any attempt to disguise or remove them.
Revealing mistake: When Private Pyle shoots himself you can see that the blood splatters from the side of the screen instead of going straight back from the back of his head.
Revealing mistake: In the early latrine scene when Privates Cowboy and Joker are mopping the floor, the mops they are using are dry. As they dip each mop into the bucket, there is no water dripping. When they move the bucket, the sound indicates it is empty, as anyone who has ever used a mop & metal bucket could validate.
Revealing mistake: We find out at the end of the movie that the sniper is a young girl, but earlier when the first shot is taken at Eightball, you can see the sniper's fingers on the fore end of the rifle - they appear to be an adult male's fingers. (01:31:27)
Revealing mistake: The dead VC in the compound where Joker reunites with Cowboy is breathing. (01:09:00)
Revealing mistake: When Joker and Raptor are in the helo heading for the front, there is a view of Vietnamese being fired at by the door gunner with an M-60 machine gun. Several seconds pass in the scene while bullets should be tearing into the ground around them, but there are no indications of bullet strikes, even as the line of fire passes over a small creek.
Answer: It was Gustav Hasford's idea. It happened in the original book that the story is based on, "The Short Timers."
Captain Defenestrator