When the Seabees are first deployed to the island, they embark in Higgins boats (no bow ramps). When they reach the shore, the Higgins boats are now LCVPs.
In the final battle scene, a clamshell bucket is seen picking up Japanese soldiers. There are legs (with feet visible) dangling from the bucket. In the next shot, the legs are replaced with tree trunks.
When the Japanese soldier is picked up in the clamshell bucket, the feet beneath the bucket are facing the opposite direction of the soldier.
In a couple of scenes, Japanese soldiers are seen pulling the pin out of grenades with their teeth and throwing them American style. Actual Japanese grenades had a compression actuator, not a pin. Typically, they would smack the top of the grenade on their helmet to start the fuse and then throw.
In some of the Japanese attacks, they are using belt-fed American light machine guns rather than than Japanese stick-fed light machine guns.
In the first landing by the Japanese, the landing craft are U.S.N. LCVPs, which are distinctly different than any landing craft used by the Japanese. Also, there are no ships offshore from which the landing craft could have come.
When the Japanese tanks attack at 1:30:19, the American "machine gunner" is wearing sunglasses.