Continuity mistake: The inmates build the wall using mortar to strenghen it. In the following scenes after the wall has been knocked down, there isn't mortar attached to any of the rocks.
Continuity mistake: In the scene where Robert Redford first assembles the prisoners in formation, right after they finish their fake salute, the shot changes to a view of Gandolfini with a reflection on the window in front of him. The reflection shows the men already heading towards the mess hall. In the next shot, they are all still standing in formation and leave only after being told to do so. (01:01:50)