Continuity mistake: When Eragon shoots Durza on the forehead with an arrow, the depth the arrow penetrated changes three times. Average depth first, near full depth next, and finally very little just when Durza fades away. Also, the arrows between shots are different, one uses flight vanes at 45º angle, the other uses opposite flight vanes.
Continuity mistake: Right before Eragon fires his exploding arrow at the attacking Urgals, there are two separate camera shots indicating Urgals attacking from both sides of the bridge - one shows Brom running from them, and the other one doesn't. As Eragon nocks the arrow, it is clear that there are no Urgals behind him and he wins the battle by firing the arrow forward.
Continuity mistake: When Eragon and his cousin are play-fighting in the shed, a large sweat stain appears and disappears on Eragon's shirt. It's there when he kicks a pile of sticks at Roran, completely gone when he picks up the stick he uses as a sword, and then it reappears at the very end of the fight.
Continuity mistake: At the beginning of the movie after Eragon fires the arrow and the egg appears, flames can be seen around the egg in the wide shot. In the close-up of the egg, these flames are gone. A moment later after the scene with Arya and Durza, the flames can be seen again around the egg as Eragon approaches, then they're gone again at the close-up (as "Based upon the novel by Christopher Paolini" appears).