Revealing mistake: Just before Mor'du appears for the first time, Merida is holding the arrow she retrieved from the woods. When Elinor picks her up, turns around and starts walking with her, the arrow disappears from Merida's hand.
Continuity mistake: After Mor'du gets crushed by a large piece of stone, and as the sun is rising, Merida removed the tapestry from Angus to place over her mother, who is still a bear, in hope to break the spell. Angus also appears to be in the circle of stones, yet, in the shot that views everyone in the circle of stones from above, Angus is nowhere in sight.
Plot hole: During the scene where Merida gives her speech on "breaking tradition", her mother, as a bear, moves silently behind the majority of the crowd, so they don't see her. Fair enough. However, Merida and her father's clan are all looking in the same general direction (towards the crowd) while she's speaking - how does no one from Merida's clan see a bear moving at the back of the room?