Visible crew/equipment: In the final scene with Anya and D'Hoffryn the edge of the backdrop (and the set behind it) is visible when D'Hoffryn walks around her. (00:40:10)
Continuity mistake: When Anya is confronting Dawn about Xander's disappearance and the demon boy leaves, he walks past Dawn and her attention is on Anya but in the next shot you see him walking away from behind and his sleeve is still in the shot as Dawn turns back to Anya.
Visible crew/equipment: In the scene where Anya is going over her wedding vows with Tara, she steps down from her dressing platform. As she steps down, part of the camera can be seen on the far left of the screen, obstructing the shot.
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.