Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: When the school-bell rings at the beginning of the episode after Cordelia talks about hitting the girl on the bike, in the first shot Willow is just starting to put her book into her backpack and is still seated. In the next shot, Willow is suddenly starting to stand up and has already put her book away.
Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: When Cordelia is talking about hitting the girl on a bike with her car and the bell rings, pay attention to Willow's hands. As it cuts from the front angle to the back angle, Willow's hands instantly change positions. (In the first shot, her hands are together, but in the next, her arms are crossed in different directions).
Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: When Cordelia raises her hand in the class at the beginning of the episode, look at Harmony's hand on the desk next to her. Harmony's hand instantly changes position and moves about six inches to her left between cuts when Cordelia is called on.
Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: This is subtle and hard to notice. When the teacher is giving a lecture in the beginning, she says "Ok, so talk to me people " and starts to turn and walk to her right. When the camera cuts, she starts to turn and walk to her right a second time. It goes by fast, so it's kinda hard to see.
Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: When Cordelia is giving her May Queen speech, Willow notices 2 body guards standing around. In both shots you can see the same 2 girls, wearing light blue and grey, walk behind The Body guards. The same girl in light blue can also be seen in several different shots, in the crowd listening to Cordelia and walking around as Buffy talks to Willow and Xander.
Continuity mistake: When Buffy drowns in Prophecy Girl, she falls into the pool with her hair tied in a ponytail and her arms under her. When she is discovered by Angel and Xander, her arms are by her sides and her hair is loose. (00:33:55)
Continuity mistake: In the episode Prophecy Girl, during Buffy's rooftop fight with the Master, her shoes change in between shots from white heels, to black boots when she's kicking him, back to white heels. (00:41:35)
Other mistake: The Master's skeleton, before it is destroyed, looks to be female and not male. (Note the wide hips and large birth canal-like hole.) Also in the Next season's "When She Was Bad," they have another shot of the same skeleton, however the hips are much smaller and more male. (00:43:55)
Continuity mistake: When Buffy falls down after the Master bites her, you can see her fall, then hear her hit the dry ground. But then a few shots later she falls again and this time into water.
Audio problem: When Willow and Jenny jump in Cordy's car, Cordy mentions that she saw "these things coming at me!" However, her mouth isn't moving for the last few words.
Plot hole: Angel says he cannot give Buffy mouth-to-mouth because vampires have "no breath." However, throughout the entire series, vampires are shown to breathe, and are able to do things like smoke, gasp, get choked out, etc. (Granted, the franchise also confirms a vampire cannot die from asphyxiation, but still they're shown to breathe regularly).
Deliberate mistake: When Willow and Jenny go out to the parking lot only to discover vampires converging on the school, pay attention to the extras. You can make out some of the same extras in different shots coming from different directions. (Most notably, you can see a woman in a long black-and-white skirt coming from the direction of the parking lot, then a few shots later, she's suddenly over in the football field coming from there.) They obviously reused extras in different shots to "pad out" the crowds.
Continuity mistake: When Buffy descends into the Master's lair, the way she is holding her crossbow changes depending on the camera angle. Sometimes it's pointed straight forward, sometimes it's pointed upwards.
Continuity mistake: When Giles and Jenny are talking about Buffy, Giles says "We must get in touch with this Brother Luca chap!" watch Giles. In the first shot he's still reaching towards the top of the cabinet. Instantly in the next shot, he's in the middle pulling something down from the top of the cabinet.
Continuity mistake: When Giles and Jenny are talking in the library, when Jenny closes the bible, the way she's holding it changes instantly between shots.
Continuity mistake: When Buffy is consoling Willow after she discovered the dead students, as Buffy leaves the room, Willow's mouth is open and she looks down. It then cuts to a wideshot. Instantly, Willow's mouth is closed and she's staring forward, and she looks down a second time.
Continuity mistake: When Jenny meets Giles after Xander's botched attempt at asking out Buffy, the way she's holding the folders to her chest subtly changes repeatedly between the closeup and wide shots.
Continuity mistake: When Buffy turns down Xander's date proposal at the start of the episode, throughout the scene, Xander has his backpack up around his right shoulder. However, when he walks away, it's instantly down by his side.
Visible crew/equipment: When Buffy meets Giles in the library after the opening credits, and they start talking, throughout the scene, you can see set lights and reflectors reflected in Giles' glasses multiple times. This happens often throughout the series, but is particularly noticeable here.
Revealing mistake: When Willow says "The usual " at the beginning of the episode and it cuts to Buffy landing on the ground, you can briefly but see the stunt woman playing Buffy's face in the first shot for a solid second or two. That definitely isn't Sarah Michelle Gellar.
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.