Continuity mistake: When Buffy and Spike are fighting on campus during the day, Xander tries to help Buffy but Spike throws him against a lamppost. When he lands in the ground he falls completely on the grass on his side but if you look behind Spike in the next shot he lies half on and half on the grass on his front.
Add timejulie sheridan
Continuity mistake: Where Willow is pulling the bricks out of the jail near the beginning of the episode. When she first pulls the bricks out of the arch thing they are from a different area than when you see the arch from the inside of the jail, even allowing it to be 'inside out' (as seen from outside and then the inside).
Continuity mistake: When Buffy is told that Angel has become Angelus, she mentions "I should have known. When I saw him at the house he was different", but her earlier conversation with him took place not at the house but at his apartment. This isn't just a strange turn of phrase - the scene was originally shot at the Summers house, but was changed to Angel's apartment for more emotional resonance.
Continuity mistake: In the episode (series six) where Buffy thinks she killed Warren's ex-girlfriend, when she's at the police station, in the over the shoulder shot of Desk Sergeant, he is holding the phone between his shoulder and ear. But in the front shot of him the phone has jumped to his hand.
Plot hole: When Buffy enters the house to rescue her mother she uses a stake to stop the front door from closing. A vampire then grabs it and the door closes. Why did Buffy use a stake knowing that there was a vampire there when she was almost killed by a vampire with her own stake earlier in the episode because Giles had been injecting her with a muscle relaxant?
Visible crew/equipment: In the scene where Willow visits Xander to discuss what may be going on with Oz; towards the end of the scene, when the camera is on Willow, you can see the sound mike at the top left of the screen next to the green crate in two different shots.
Plot hole: When Buffy and Sheila are walking out of school after the meeting with Snyder, Xander runs up to talk to them. He mentions during this scene how Buffy will be helping out at the Parent-Teacher Night, but he shouldn't know this - Buffy only just found out about it in the meeting with Snyder.
Other mistake: We're shown a variety of photographs and pictures of different members of the gang at one point. One of those, the one of Xander and Anya, is actually a shot from the episode "Something Blue". I doubt very much that during the events in that episode, someone conveniently decided to take a photograph.