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: His Shadow does "the kiss" and transfers his soul into the new host with the Clerics. As they are lifting the new host back up, the shot cuts and suddenly two Clerics appear behind him. (00:16:10)