Other mistake: As Spike follows Buffy into the hallway by her front door we see Spike's reflection in the mirror. Vampires do not have reflections, so Spike should not have been visible.
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.
Other mistake: The newspaper that Xander is reading in the library has the same text over and over again and does not at all match the headlines. (00:18:05)
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.