Lies My Parents Told Me - S7-E17
Revealing mistake: When Spike tells his mother that he's become a vampire, she asks him if he's drunk. He responds with "Little bit." Just after he says this, someone can be heard coughing off-camera. It could be his mother because she isn't supposed to be well, but it seems odd that the camera wouldn't be on her during this. And a cough wouldn't really be necessary for the scene.
School Hard - S2-E3
Revealing mistake: When Spike is talking to the Anointed One for the first time, one of the henchmen tries to rush him from behind and Spike backhands him without even looking, but Spike's hand movements, the reaction of the henchman and the sound of the punch connecting on the audio track are all out of sync.
Dirty Girls - S7-E18
Revealing mistake: In the opening scene, where Caleb picks up the potential slayer in his truck, there is a brief shot of the two from the hood that pans slightly to the right. In doing so it reveals the edge of a screen showing a fence and trees passing by, revealing that the car is in fact stationary and in a studio. (00:02:40)
Revealing mistake: In the scene where Absalom is speaking of the Anointed one, the line can be seen where the vampire makeup meets up with his real face. This may be seen in various scenes he is in.
The Harsh Light Of Day - S4-E3
Revealing mistake: When Buffy is fighting Spike, she throws him against a table and then runs over it; you can see this is her body double. It is extremely obvious.
Life Serial - S6-E5
Revealing mistake: The entire scene where Buffy is experiencing accelerated time, shadows of trees move in the wind but never actually grow or shrink. Even over short increments of time, the sun can move shadows noticeably, and judging from the speed of the people around her, Buffy is in accelerated time for at least half an hour. (00:08:25)
Revealing mistake: Drusilla's reflection can be seen in the metal part of what appears to be the base of a shopping mall map just as she's getting up after the Judge has been blown up.
After Life - S6-E3
Revealing mistake: When Dawn is possessed and breathing fire the camera changes and we can see the safety tube the fire is coming out of. (00:23:55)
Revealing mistake: As Willow walks through the forest pursuing Warren, just after we first see Warren running, there's a shot of Willow walking into a bit of a clearing. Her shadow crosses a tree on the right (could be caused by the moon), but then a couple of seconds later another shadow is visible on a tree to the left - obviously an artificially lit forest. (00:33:15)
All The Way - S6-E6
Revealing mistake: In one fight scene, Buffy uses a car door to block a kick from a vampire. The window of the car breaks before the kick connects with it. In fact, it breaks while the foot is still in mid-air. (00:37:00)
Revealing mistake: In Graduation Day Part One, when Faith shoots Angel with the longbow, you can see the false point spring out as if it were on a hinge from inside his jacket. (00:22:47)
Revealing mistake: Throughout almost the entire sword fight between Buffy and Angel, from the far away shots, its clear that the actors are stunt doubles. It's very visible when there is a shot from the top. (00:34:05)
Revealing mistake: Towards the end when Oz says to Veruca, "You don't wanna find out what I am", and partially turns into a Werewolf. You can tell that both of the actors were filmed separately for the CGI's sake as Oz stands out from the picture more than Veruca does. The same effect is obvious during "Doppelgangland" in season 3 where there are 2 Willows onscreen at once.
Lover's Walk - S3-E8
Revealing mistake: Early on, Spike falls down drunk. He wakes up because his hand is on fire from the sunlight. When he runs away, a little bit of fire stays on the stones where his hand was when it started.
Revealing mistake: When Tara is being chased by the monster and she falls, she says a spell and white smoke comes out of her hands but it is coming out of the sleeves of her shirt, the way she holds her hands up suggests this as well.
Revealing mistake: In Xander's dream, when he climbs into the back of the ice cream van, for a split second before the window goes out of view the backdrop cuts out and the blue screen is visible. Only visible in widescreen version.
Revealing mistake: Right at the beginning of the episode just before we see Warren squirt Jonathan with water, we see Buffy on one of their security cameras. The footage of her is pre-recorded as the progress bar for QuickTime Media Player is visible at the bottom of the screen. The window was moved down to hide this fact, but not far down enough to hide it completely.
Revealing mistake: Near the end when the good Willow says goodbye to the evil Willow and hugs her, there is a shot of the back of the good Willow's head and the evil Willow's face. You can tell that the good Willow is a body double wearing a wig - there's something circular near her scalp, possibly an elastic band.
Chosen answer: From what we see in the series, certain girls are identifiable as potential Slayers - Kendra clearly fell into this category. The identification method is presumably mystical in nature, but the Watchers' Council are pretty effective at that sort of thing, so they're quite good at tracking down the potentials ahead of time. Not perfect, though - it does appear that Buffy herself may have slipped through the net - certainly she had no inkling of what she was until she'd already taken on the role of Slayer. It is possible, however, that this was actually cultural - an American family would hardly be likely to turn over their daughter to some strange man for 'training', so the Watchers might have chosen to keep an eye on her covertly, whereas some other cultures (like Kendra's Jamaican parents) might be more willing to believe.
Tailkinker ★