After Viktor and his personal guards drop into the battle scene towards the end of the film: the zoom shot from Lucien's perspective shows Viktor holding his helmet under his arm. Viktor then puts the helmet on and enters the fray. The camera cuts back to a distance shot from Lucien's perspective. In this distance shot, Viktor's helmet is back under his arm and he is standing where he was in the zoom shot.
Several of the slaves that were bitten by one of Viktor's Lycans take part in the assault on the castle, although they were bitten at least 3 days before they do not change in the light of the full moon. In the original Underworld film it was stated the only the older Lycans could change at will and resist the influence of the moon.
After Lucian's final fight with Viktor, he steps out on the ledge above all the Lycans. They are all clothed like they were before the battle started. When they transformed into their wolf forms, all their clothes would have been ripped off.
When Lucian arrives near the room where Sonja is being held captive, we can see two guards at the door, both right handed (sword at left side). When one of the guards walks away, we can see that he is is left handed (he has his sword at his right side and holds it with the left hand).
Just before Lucian tears up the grate to go to Sonja, he is watching the vampires unceremoniously shove the Lycans around. In the first shot of him, his hair is completely out of his face, then it cuts back to the Lycans. In the next shot of Lucian, his hair is very in his face. Then, after another shot of the Lycans putting down axes and such, it goes back, and Lucian's hair is completely out of his face again.
Sonja's eye color keeps changing back and forth between brown (Rhona Mitra's natural eye color) and whitish-blue (contact lenses). The vampires' eye-colors change throughout the movies in the first two films as well. They are often brown (natural eye-colors of the actors/actresses), but change to blue when the intensity of a situation increases. For example, Selene's eyes change from brown to blue during her fight/searching sequences in both movies, as well as her sex-scene in the second film. Kraven's eyes exhibit the same behavior in the first two films.
At the beginning, when Sonja is riding out to greet the human nobles and receives her "sharpened" sword from Lucian, you can see she doesn't know how to properly wear/carry the sword while on horseback and the sword actually falls out of it's scabbard. There is a quick cell-change as this happens and the sword magically puts itself back in the scabbard and rights itself.
When the offers of silver are brought before Viktor, it's quite apparent that they're hollow as the actors can carry them with ease. Silver is a very dense metal (10.49g/cm^3) and even if the silver of the coins was watered down, the boxes would each weigh in excess of 50kg (or 110lbs - the weight of a fully grown [and fairly slim] woman).