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)
Revealing mistake: When Buffy sets the vampire alight, his face suddenly gets very shiny, showing the fireproof gel that was applied to the actor's skin. (00:37:20)
Revealing mistake: As Willow, Cordy, Miss. Calendar and Giles are wheeled over the bones of the Master, Giles goes past and his hand brushes against the bones - the skeleton bends, revealing that it is rubber. (00:36:10)
Revealing mistake: In the scene where Absalom is speaking of the Anointed one, the line can be seen where the vampire makeup meets up with his real face. This may be seen in various scenes he is in.
Revealing mistake: When Buffy is fighting the vampires that are trying to resurrect The Master, at one point she is grabbed from behind by a male vampire and kicks a female vampire in a green top twice (right after Absalom shouts "The sacrifices! Stop them!"). If you watch closely, both of the kicks miss the vampire, revealing that the fight was choreographed. The punch that she delivers to the male vampire afterwards also looks fake.
Chosen answer: He had larangitis and because of the shooting specifications they had to have him shoot the scene with a bad throat, they then re-recorded his lines and dubbed them. At various points during the scene and indeed the episode you can see him touch his throat or rub it.