Seeing Red - S6-E19
Continuity mistake: Shortly after Spike tries to rape Buffy, he returns to his crypt and drinks a glass of vodka. Clem then arrives to watch television with him. During Spike and Clem's conversation, the position of the bottle of vodka constantly changes between shots.
Seeing Red - S6-E19
Continuity mistake: When Buffy is fighting Warren at the end, there are several shots when it's clear that it's a stuntman playing Warren. The stuntman is taller and slimmer than Adam Busch, and the shape of his face is different, as is his hair in the front.
Seeing Red - S6-E19
Continuity mistake: When Tara got shot, the blood on Willows shirt was spread out, but in the next shot, there were big clumps of blood.
Seeing Red - S6-E19
Continuity mistake: At the beginning of the episode, Buffy is running from the large saws and jumps out the door, she sits up and you can see the book is fully intact and in one piece. After the opening credits Tara picks up the same book and it falls apart from being cut in half.
Seeing Red - S6-E19
Visible crew/equipment: When Buffy says, "Goodnight, bitch" to Warren, right before she kicks him it is quite obvious that it is his stunt double.
Seeing Red - S6-E19
Other mistake: At the Bronze, when Warren punches Xander the first time, Warren never actually punches him, and Xander starts to go up in the air before you even hear the sound of the punch.
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.