Continuity mistake: After the Battle of Bull Run, Jonas Steele is seen surveying the aftermath. Trouble is, there are a heck of a lot more dead bodies lying around outside the house than there were when the battle was raging, including those of Union soldiers. As the scenes of the battle demonstrated, the Union never actually got as far as the house; all their casualties were incurred on the slopes approaching the house, and then they retreated.
Other mistake: A few times during the whole show there are certain shots where the corners of the screen are blacked out, as if a circular lens cap has been fitted to the camera. There seems to be no logical reason for this, therefore it has to be a mistake. One example is when Jonas is leaving his wife for the last time and he says 'there's a big battle brewing'.
Revealing mistake: When the coward is being branded using a red hot poker, you can see that the metal 'C' hasn't been heated at all when it is brought next to his face. Compare it to when they tested it on a piece of wood beforehand. The poker is also covered in a really fake-looking red paint.