Revealing mistake: As Willow is raising the satanic temple from the earth, we can see the support cables holding the temple upright from behind. This is plainly visible in the bottom right hand corner, complete with bright orange clasps on the temple itself.
Continuity mistake: During the opening of the season 6 episode "Grave," Willow's nose starts bleeding and she gets blood on her upper lip. Over the next several shots the blood consistently keeps appearing and disappearing.
Visible crew/equipment: When Xander is leaning over the hole in the cemetery into which Buffy and Dawn have fallen, he causes the edge of the hole to collapse. Seen from below, everything is fine, but the shot from above Xander shows a hand grabbing hold of the back of his trousers and his briefs are visible.
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.