Pvt. Cliff early in the film is seen using a Thompson sub-machine gun. Later, on the patrol, he carries a M-1 carbine. Marines do not simply change weapons.
Marines often responded to officers with "Yes, sir," rather than "Aye, sir," or "Aye, aye, sir."
A Japanese sniper in the top of a palm tree controls a dummy in another palm. He gets two marines to shoot at the dummy (which knocks the dummy out of the palm). The two marines then rush over to the fallen dummy and discover that it is not the sniper, but a dummy. The sniper then kills the two marines with two shots (one immediately after the other). BUT the sniper is using a bolt action rifle, and his two shots (to kill the two marines) are much too close together for the sniper to have fired with the bolt action rifle.
The Japanese have changed their uniforms to that of the Marines in order trick the Americans. When they are being strafed and bombed by American aircraft, there is a shot of 40 Japanese being shot. When they fall you can see that the extras in the back ground are wearing blue jeans and sneakers.
The "Japanese" aircraft are not Japanese attack aircraft. A second side view as they were departing showed them to be Mooney M21's. Their distinctive tail shows this very clearly.
After the beach landing, during the penetration into the underbrush, TWO marines throw grenades (one each for a total of two) into a machine gunner's nest. But there is only ONE explosion.
When the marines advance from the beach, a brief wide shot reveals a paved road and a well-manicured hedge in front of them on the right side of the screen.
Pvt. Egan is always eating cans of beans using white plastic spoons. Up until Vietnam only metal spoons were used.
As the opening credits segue into live action Sergeant Honeywell turns to face his men from the upper deck of the transport ship. At the upper left of the screen, next to the marine with the bazooka, you can see the edge of the board on which the final credits were painted.
The American tanks are portrayed by M41 Walker Bulldogs, which were not developed until after the war.
Some of the flashback scenes, in particular those featuring Captain MacDonald's wife, Julie, are unrealistic in their depiction of the time period intended - namely early to mid-1940's. In particular, she is seen wearing her hair and mascara in a style that is clearly not of the mid-1940's, and in one scene, is shown wearing a blouse that is clearly of a mid-1960's design.
When the Japanese colonel is watching the failure of his troops' final attack, the field glasses he is using are of a modern design and have a plastic strap.
The flashbacks of the American Marines are all wrong. All the clothing, hair styles and make-up are clearly from the time the movie was made, the mid 1960's. This is likely due to budgetary constraints.
At around 59 minutes Sgt. Honeywell (Rip Torn) is telling the captain about what he will do to the Japanese, his lips are out of sync with the sound.