Continuity mistake: In the scene where Willow and Tara are at the old High School looking for Riley, Willow uses magic to light up the hallway. Tara then asks Willow how she did it. As Willow tells her that she taught her, she tucks her hair behind her right ear. Then after Tara says, "I taught you teeny Tinker Bell light", Willow's hair is hanging back down over her right ear.
Revealing mistake: When Spike is playing with the Buffybot in his crypt and has her throw him onto his bed to stake him, the shot of him landing on the bed and the Buffybot climbing on top him is portrayed by stunt doubles. The shot is then interwoven with another shot of the actual actors. But it's very obvious due to the poor editing.
Continuity mistake: Huge continuity gaffe. When Anya and Xander see Spike and the robot in the graveyard it is nightime. Xander goes back to speak to Spike almost immediately and sees Glory's followers kidnapping him. He wakes goes back to Buffy's house and from there they, again almost immediately, go looking for Glory's house - and it is the broadest of broad daylight. You could see it was still night before they left through the windows of the house. It also begs the question of how they got Spike from Glory's house back to his crypt in the sunlight since they had no blanket.
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