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 (2003) it was stated the only the older Lycans could change at will and resist the influence of the moon.
(at around 42 mins) After his group of slaves is sent to be bitten by the captive Lycans, Raze is the only one of his group back in his cage without a collar, but no point is made of this.
(at around 1h 21 mins) 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.
(at around 1h 17 mins) 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.
At around the 47 minute mark, a vampire is seen dead on the floor of the cell where Lucian was held. The vampire's eyes and mouth are closed. The camera cuts to Viktor and when it returns to the dead vampire his eyes and mouth are open.
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 around 18 mins) 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 its scabbard. There is a quick cell-change as this happens and the sword magically puts itself back in the scabbard and rights itself.
(at around 14 mins) After Lucian and Sonja have sex, there is a shot of Sonja lying down and Lucian is behind her. Both of them are naked, but Sonja appears to be wearing jeans visible on the left side of the screen.
(at around 32 mins) 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).
(at around 57 mins) The "blood" that Viktor is drinking shortly after imprisoning Sonja is quite clearly tomato juice or something similar - blood is not orange, and is opaque but not cloudy with sediment.
(at around 33 mins) When the Noble gets thrown into the pillar and lands with a head wound, the wound shakes after he is motionless. Also the color is incorrect for the scene given the lighting.