Continuity mistake: When Dawn was at the house, she had a grey top and a red skirt, but outside, she had a red top and black pants.
Continuity mistake: In the alleyway scene after the real Buffy kills all the Hellions (biker demons) and runs off, one of the Hellions can be seen getting up behind Xander. We then see a closeup shot of the Hellion opening its eyes while still lying on the ground and then they stand up and sneak up behind Xander again.
Continuity mistake: When Spike is looking out the window at the demons rampaging in the street outside Buffy's house, Dawn can be standing behind him with her hair trailing down below her breasts. In the next shot, when Spike pulls her away from the window, her hair is pulled back behind her shoulders despite her not having moved. (00:53:10)
Continuity mistake: Buffy's shoes change from being nice shoes, to runners (e.g. When Buffy climbed out of her grave, she had nice shoes, but when she jumped over the fence, she had runners on).
Visible crew/equipment: When Buffy and Dawn are at the top of the metal tower, and Buffy is still very dazed from being called back, there is an overhead shot with a piece of the tower falling down; if you look left of the falling piece, you can see a guy wearing a hard hat walking about.
Continuity mistake: When Buffy sat down on a car and set off the car alarm, she didn't have any dirt on her hands and only a little bit of blood, but in the next shot, there was a lot of blood and dirt on her hands.
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 ★