Continuity mistake: Near the start, after Lucian saves Sonja from the werewolves. Before Sonja throws Lucian the the sword, Viktor and his guards are walking around the corner next to the ladder. When she throws it they disappear, when Lucian catches it they reappear. (00:06:30)
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: After Viktor has bitten Sonja and she is lying on her bed, Viktor walks out of her room yelling "She will not leave this room", you can see Sonja's necklace chain on her throat and shoulder. In the following shots, hair is covering the chain entirely.
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.
Continuity mistake: In the beginning, during the meeting, when Coloman says "The nobles are upset, my lord", while walking around with his hands down. When it cuts to a close-up he is suddenly folding his hands.
Chosen answer: He's badly injured, so he goes into the hibernation chamber to heal. Doesn't mean that he's staying in there for hundreds of years; he could easily emerge as soon as he's healed up and kill Selene's family off.
Tailkinker ★