Continuity mistake: In many historical flashbacks, most notably Spike's and Anya's, hair and wigs differ greatly from previous flashbacks or episodes. The director even discusses it during the commentary on Lies My Parents Told Me, saying that he doesn't understand why the wigs aren't consistent.
Continuity mistake: Shortly after the opening credits we see Buffy, Dawn and Xander driving to the new high school. As they're talking, the rest of the credits roll by onscreen. Just after "Producer: Marc David Alpert" is shown, there is a side-angle shot of the car and you can see Xander's left arm resting outside on the open door window. But in the following shot he has both his hands on the steering wheel.
Continuity mistake: In episode 'Same Place, Same Time', Gnarl cuts Dawn across the stomach paralysing her and ripping her shirt, but when Buffy and Xander are carying her home there is no tear in her shirt or any blood. Also when Buffy goes back to kill Gnarl he tears her shirt but seconds later the rip has gone.
Continuity mistake: When Anya is sneaking into Spikes room after she closes the door the shot goes to spike and the sheet is lying square across his stomach and the camera goes back to Anya she then walks over to the chest of drawers and we see spike in the shot and the sheet is now lying at an angle so we can see his hip.
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Continuity mistake: When Buffy is discussing the Turok-Han with the potential Slayers in her dining room, one of them says that it almost killed Buffy. Rona then spreads both her arms out and says, "What chance do we have?" In the following shot, her left arm is down by her side and her right arm is resting on the dining room table.
Continuity mistake: Spike tells Buffy about drugs the Initiative would give him when his chip started firing after he was first captured. This is wrong in two ways: firstly, we saw most of Spike's time in captivity, and he escaped before they could drug him, and secondly, he didn't even know he had a chip until he escaped and tried to kill Willow.
Continuity mistake: When Willow is out with Kennedy, Willow tells her that she doesn't get why she likes her. As she says "I still don't get it", she pulls her hair back over her left ear. Then when she says "Why you like me" in the following shot, her hair is hanging back down over her left ear again.