Continuity mistake: Near the start of 'The Wish', the scene where Cordelia is talking to the Cordettes and meets Anya. During the close-ups of Harmony, Anya is standing to camera right of the brunette Cordette behind her. A few seconds later when the shot is further away Anya is standing camera left of the Cordette.
Continuity mistake: When the female watcher is being destroyed after Buffy cuts off her arm. Her sleeve is empty, which is not possible, as Buffy could not have removed the arm but not the sleeve. Also the arm on the floor is removed at the elbow, but the watcher has lost the entire arm in a previous shot.
Continuity mistake: When Buffy is walking forwards through Angel's dust in 'The Wish' there's a vampire running towards her, only a few steps behind. Then there's a shot of her walking forwards taken from another angle, then when we can see behind her again the same vampire hasn't moved forwards at all, he's still running from 3 steps behind her.
Continuity mistake: During season 7 we are told several times that the First cannot take corporeal form, ie. it cannot touch things. However, in its first appearance in the season 3 episode 'Amends', when it manifests as Jenny Calender and runs its fingers through Angel's hair you can see his hair moving underneath her fingers. It wouldn't be able to do this if it weren't corporeal.
Continuity mistake: In the scene when Spike was in Buffy's kitchen with Joyce, Buffy keeps him down on the table and grips Spike by the throat. She uses her left hand (in the close shot it is also the left hand) but in the next scene when Angel is also in the kitchen and Spike hits Buffy's hand, she uses her right hand (Spike hits her right hand).
Continuity mistake: When Joyce throws the bag with the stray cat into it into the hole, the bag lands with the knot at the top right, and shows no signs of moving. They chat for a few seconds about something to say, then when Buffy starts filling in the hole (with no sound of a moving bag), the bag's now lying with the knot on the left.
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Continuity mistake: In 'Graduation Day, Part 1' when Buffy and Faith are fighting Faith falls into a table, knocking all the comics on it onto the floor. In one or two later shots you can see (on the very edge of the screen) that there's still a comic on it.
Continuity mistake: At the beginning of the episode when Buffy is fighting the demon, she stabs it and the demon lands half on top of her. When they fall to the ground, you can see a big stain on her pants where the demon touched her. But when she stands up, her pants are clean.
Continuity mistake: After Xander and Oz's "Kryptonite" conversation, Buffy tosses a book onto the table and stands up to leave. The book lands partially on top of another book and slides almost off it. In the next shot the book is lying on top of the other book perfectly.
Continuity mistake: In Graduation Day Part 1, when Faith and Buffy are fighting, Faith throws Buffy into the wall near the TV, but the TV is intact. A few shots later, there is debris on the floor, glass and such, from the TV, and when it's shown again, the TV is unbroken. This happens a few times. In Graduation Day Part 2, when Faith and Buffy are talking in the apartment, the TV is broken again.
Continuity mistake: After Buffy slams the Gavrok spider thing on the ground with her back, she gets pulled up by Angel. We see this shot from the back, and we see that the Gavrok is no longer on the floor or on her jacket. In the very next shot, we see Angel kick it across the floor. Where did it come from?
Continuity mistake: Whenever we see the two Willows together in this episode we see that Vampire Willow is understandably very pale in comparison to the living Willow. However, in the library with Cordelia, when Vampire Willow says 'How about dinner?' her face is no longer pale.
Continuity mistake: When Buffy wakes up in the hospital bed you can you can see the black strap of her tank top underneath the gown. Then when she gets out of bed on walk to Faith, and it shows her walking through the hall you can see a white spaghetti top through the gown.
Continuity mistake: In the episode 'Anne' when the latest people to be brought into hell are lined up with a demon talking to them, there are 3 or 4 people standing between Buffy and Lily. When the demon asks them all in turn 'Who are you?' we see him asking Lily and after that the shot rests on Buffy, but in the background we only hear him ask one more person before he gets to Buffy.
Continuity mistake: In numerous episodes, including 'Homecoming', we see vampires being killed when they weren't stabbed through the heart (the Texan girl vampire in this episode is stabbed practically in the stomach). But the show also makes an issue of the fact you have to stab a vampire directly in the heart to kill it (think the flashback in 'the Becoming, Part 2' where Buffy misses the vampire's heart, or 'Pangs' where Spike gets staked several times).
Continuity mistake: When the detective is interviewing the woman in the alley, he mentions that the murder took place "last night". In "Bad Girls", we see Buffy visiting Faith the day after the murder, and that night is presumably when she has the dream about drowning, so by the time the detective is asking this question it is actually two nights after the murder, not one.
Continuity mistake: When Wesley and Cordelia kiss, after a few seconds they freeze in place. Cordelia has her left arm around his waist, we see a shot of her face for about a second with her arm definitely angled downwards, then we cut to Wes' face and her hand's now on his neck.
Continuity mistake: In the scene where Xander and Willow are stuck in the factory Xander wakes up you'll notice that the blood on his side burn closest to his ear constantly changes lengths from shot to shot. It goes from under the chin to slightly below where the blood on the other side of his side burn is.
Continuity mistake: In 'Consequences' Buffy says to Faith 'Less that 24 hours ago you killed a man'. In 'Bad Girls', Faith kills Finch in the alley on the first night. One night later Buffy and Angel are attacking Belthazor, the night after that is the night when Buffy dreams about drowning, so the night when Buffy says this is actually about 72 hours after the murder.
Continuity mistake: In 'Graduation Day, Part 1' when Giles stabs the Mayor, as he takes the foil out of his chest you can see a wound on his shirt. When the shot changes it has disappeared. I know the Mayor is invincible and his wound heal in seconds, but it still takes a second or two for his wounds to close up, they don't just disappear.