When Edward wakes up after Isaac knocks him out, Isaac holds his gun without his thumb on the hammer. The scene cuts between him and Edward, and with every scene, the position of his thumb changes.
In an early scene, a shirtless zombie walking through a pond washes off the make-up on his forearms.
The rifle used by Edward Young would not have been used by a confederate soldier, especially not as frequently as depicted. Repeating rifles were only adopted by the Union in very low numbers; while they were occasionally captured by confederates, they didn't have the ability to produce ammunition for them.
The confederate flag seen in the movie was not used during the civil war.