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.
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.