Factual error: Throughout the first battle at "Hacksaw Ridge" some of the men are seen pulling the pins of grenades with their teeth. In actuality, the pins on the grenades (both modern and old ones) take at least 7 pounds of force to release the pin. If someone did that, they'd either cause severe damage to their teeth or maybe lose a couple.
Factual error: When Desmond's dad interrupts the trial, he is seen with the Silver Star medal. Desmond's father served in WW1, but the Silver Star wasn't created until 1932.
Factual error: Once the men first climb up to "Hacksaw Ridge," the camera pans over numerous corpses. One of them has an M1 Helmet lying on its top to where the liner is seen. The liner has the web suspension straps cross each other instead of being tied in the center. This means the liner is a Vietnam era type, and not World War II.
Factual error: When Desmond's father is in his WW1 uniform, to the far right he is wearing a French Croix de Guerre (Cross of War) - however, this decoration incorrectly hangs from a WW2 type ribbon.
Factual error: During the whole battle scene there is not one single soldier running out of ammunition, other than by implication (in hand-to-hand combat for example you could assume that they had run short of rounds). We do not see a single shot when ammunition actually is running out, for example by the clinging ejection of an empty M1 rifle magazine or by machine gunners shouting for ammo. No one ever seems to reload. When the sergeant is dragged by Doss, firing his grease gun, he had already fired at the sniper without reloading and then fires a continuous burst of at least 100 bullets. The magazine of a grease gun contains 30 bullets.
Factual error: When the Japanese commander is committing seppuku near the end of the final battle, after he makes the first cut his second cuts his head clean off, and it can be seen falling to the ground in front of his body. While having a second perform this act was indeed commonly part of the ritual, the second was actually supposed to ensure that the neck was not cut cleanly through, i.e. He was supposed to stop short and leave the head attached to the body by a small flap of tissue. It was considered extremely undignified and unnecessarily messy for the head to come clean off and roll around on the ground. For that reason, it was important to choose a second who was a skilled swordsman.
Revealing mistake: When the stretcher is being lowered down on the line, the high tension steel wires can be seen under the hessian rope.