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.
Factual error: There is a scene where all the prisoners, wielding their metal dinner trays, gather together to form a blockade around the trebuchet, using the trays as shields. In this scene, Colonel Winter is overlooking the prison yard and it's goings on from his above office. General Irwin looks up at Colonel Winter and gives a challenging smile, which is then returned by the Colonel. However, how could he have even seen anything, let alone Irwin, with all the sunlight that was being reflected in his face by the metal dinner trays? Winter's face was completely lit up.