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: Throughout the battle scenes on the ridge, the machine gunners can be seen picking up the guns to fire on the move. While not impossible, not as painless as it is made to appear. The M1919 .30 caliber air cooled machine gun was light enough to carry. But for the gunner to hold an already hot barrel and continue firing would have been extremely painful without the issued heat retardant mitten or the barrel "hoop" w/ wooden handle. These fixes allowed the gun to be fired on the move and saved many from having their palms burned and scarred for life.
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: The soldiers somehow have cool looking, perfectly fitting dress uniforms, on the day of their enlistment or before they've gone to boot camp. Not something they've have so quickly.
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.