Continuity mistake: In the scene where Willow and Tara are at the old High School looking for Riley, Willow uses magic to light up the hallway. Tara then asks Willow how she did it. As Willow tells her that she taught her, she tucks her hair behind her right ear. Then after Tara says, "I taught you teeny Tinker Bell light", Willow's hair is hanging back down over her right ear.
Continuity mistake: When Buffy, Willow, and Dawn are discussing Riley's problems in the bedroom, Dawn's hair shifts from over her shoulder in the long shots to behind her shoulder in the closeups. (00:16:45)
Other mistake: Towards the end of the episode when Spike tells Buffy to stake him and removes his shirt, you can see that he has a tan on his upper back/neck to his hairline from wearing a t-shirt.
Visible crew/equipment: When Harmony sits down opposite the operating table, behind her in the glass there is a reflection of a boom mike.
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 ★