Continuity mistake: When the dwarves finally decide to leave Erebor and join the battle, they swing a giant gold pendulum into the blockade they all made earlier in the film, knocking it over to create a causeway, across which they all then run in a battle formation, straight into battle. However, just as they all exit the keep, the pendulum is on its downward swing back outside of the wall. In all following shots (from numerous different camera angles) the gold pendulum simply disappears. This is most notable in the shot taken directly in front of Thorin as the dwarves run towards the camera. The pendulum should be visible in the large hole in the wall behind them, but instead the hole is empty, with no pendulum to be seen.
Revealing mistake: During the battle of Dale, Bard confronts Alfrid, who is dressed as a woman and is trying to escape with looted gold. During the confrontation a troll climbs over the parapet and is killed by Bard. As the troll falls, Alfrid also falls. When he lands, the ground near his hand ripples like fabric covered in sand.