After Life - S6-E3
Visible crew/equipment: When Dawn is possessed by the demon and breathes fire in the magic shop, in the bottom right corner of the screen in one of the shots, a metal tube can be seen blowing fire onto the set when it should be coming from Dawn's mouth. A hand can also be seen holding a blow torch, igniting the tube and then quickly pulling out of shot.
After Life - S6-E3
Continuity mistake: When the demon is attacking Buffy in her room right after Willow's spell has made it corporeal, there's a shot from behind of it drawing back its left hand, followed by a shot from the front of it delivering the backhand with its right hand. (00:34:40)
After Life - S6-E3
Continuity mistake: When Anya was possessed, she cut her right cheek, but in the next shot, her left cheek was cut and she was cutting her right cheek again.
After Life - S6-E3
Continuity mistake: When Dawn and Buffy were talking in Willow and Tara's room, Dawn's necklace went from being above her top, to underneath her top.
After Life - S6-E3
Continuity mistake: Watch Spike's hand when he punches the wall. The punch lands with his middle knuckle in the wall, but when he looks at his hand it is mostly the largest knuckle that is bleeding. (00:24:55)
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)
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 ★