Revealing mistake: When all the vampires are saying "for the dark", and one gets staked, there's a mark on his shirt where the stake's about to come through before it actually happens. Plus Buffy's hands are in the wrong place for her to have pushed that stake through, as well as there being no stake in her hand when the vampire gets dusted. Not only that, but the mark isn't even close to the vampire's heart to begin with. (00:34:50)
Continuity mistake: Kai is flying his dragonfly space craft towards an enemy ship in the beginning with his wingmen as they sing. Kai's shadow is cast on the wall behind him on his left, but when the camera cuts to a close up of him, his shadow is suddenly on the other side. (00:02:05)