Continuity mistake: The scene in which Sonja is executed in front of Lucian is incredibly different from when the same scene was portrayed in the original movie as a flashback. Obviously they had to reshoot it to account for the new actress portraying Sonja, but not only is the basic setting visibly different, but important details of the scenes have been altered as well. Ex. In the flashback scene in "Underworld", there's a giant crowd of slaves in a cage behind Lucian, and Sonja screams and slowly burns up. In this film, the crowd of slaves is not there, and Sonja instantly burns up without screaming - in fact, she's rather calm and trying to calm down Lucian. You can't argue that Lucian's memory of the events have changed over time, as the "Underworld" movies have made it clear that immortals have impeccable and essentially perfect memory (more-or-less genetic memory) that can even be absorbed if their blood is consumed by vampires.
Continuity mistake: In the beginning when one of the guards are dragging a prisoner, on of the guards looks in to the cell with the baby Lucian and the werewolf. At first when we see the guard look in through the bars, he face is totally dark. When it cuts from inside the cell, his face is suddenly lit up.
Continuity mistake: In the stand-off scene between Victor and Lucian, Victor jumps out the castle pillar window with his guards to the castle floor. Lucian is standing on the wall with the lycans. Victor and him look at each other, and in the first look, Victor has no helmet. Then he puts it on. Then they look at each other again and the helmet is off. Then they show Victor again with it on, fighting lycans with a sword.