Factual error: In the third chapter of the movie, as the Union soldiers run into the Crater, they are flying the fifty-star American flag which is not the correct flag for the time period. The flag, if it is counting all states, even Confederate states at that time, should have 35 stars. A star for West Virginia, which seceded from Virginia during the American Civil War, was added to the flag in 1863.
Visible crew/equipment: During the scene when Inman is ploughing his lone furrow he is not quite alone. In the final shot, when he is in the foreground and Ada is playing the piano on the horse-drawn wagon in the background, a pair of hands and arms can be seen at the left hand edge of the screen guiding the horse which is pulling the plough.
Other mistake: When the home guard are trying to force the Swanger boys out of hiding by crushing Sally Swanger's fingers in the fence, it appears to be only her fingertips that they've trapped. Yet when Ruby and Ada come to help her (a couple of scenes later), as they pull the fence up, the majority of her fingers come out from the fence.
Factual error: Women of rural areas in 1864 did not shave their legs to the extent shown throughout the movie. For example, when Inman is seduced by the man's wife and when he's having sex with Ada, you'll notice both women are very clean shaven. Even if they had shaved, they didn't have such precise shaving tools back then.
Audio problem: During the scene where Ada and her father are talking about the farm, and how they will manage come winter, Ada says : ...."But with no-one left to work this place, nothing to buy, nothing to buy it with".... her mouth only starts moving to what she is saying half way through the sentence.
Continuity mistake: In the scene after the battle at Petersburg, Inman is guiding his wounded young friend out of the crater back to their camp. Next to the road to the camp you see several big piles of dead Union soldiers, probably to show how bad the battle turned out for them. But wouldn't it be logical if the Confederate soldiers first saved their own wounded soldiers, and only then arranged the bodies of their dead enemies? Inman was very concerned about his young friend and was still in good state after the battle, surely he would have gotten him out a lot earlier after the battle.
Continuity mistake: We see Oakley on a stretcher after the battle in the crater and he is talking to Inman, and then we see him in a tent (before he dies) asking Mr. Thewes to play him "something sweet". In these two scenes, the blood and mud on Oakley changes entirely. There is no reason it should change in such a short interval.
Factual error: Inman appears to be hospitalized in some capital in late summer of 1864. The capital cannot be Richmond, which is under siege, so it must be Raleigh. In the hospital's yard are well-dressed ladies having tea and slaves picking cotton, both of which are highly unlikely in the final months of the war. Raleigh is quite a distance west of the North Carolina coast, and Cold Mountain is at least 250 miles almost due west of Raleigh. Yet, Inman went east to the coast. In the very next scene after wandering on the coast, he has somehow arrived in the mountains, which are less than 50 miles from Cold Mountain in Haywood County. Thus, the majority of his adventures had to have taken place in the last 50 miles of his quest, which is not what is implied by the plot.
Revealing mistake: When the musicians are hiding out on the mountain, "Georgia" finds a frozen deer which he proudly displays to the rest of the gang. The beast is frozen stiff which is why it doesn't flop around but it also seems suspiciously light too. I reckon an animal of the size depicted would weigh around 30 kg - no way he could've held it out in his arms like he does.
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Other mistake: Inman yells at the escaping slaves because he wants to buy an egg, and then spits blood from his throat afterward and we can tell that it hurts him to speak but then we see him in the swamp and he yells at the minister, but his voice does not crack and it doesn't seem to hurt him. Then when they are escaping in the ferry, he rubs his throat from talking. It should have hurt his throat badly when he yells "I should have shot you." but his throat seems fine.