Continuity mistake: Spike tells Buffy about drugs The Initiative would give him when his chip started firing after he was first captured. This is wrong in two ways: firstly, we saw most of Spike's time in captivity, and he escaped before they could drug him, and secondly, he didn't even know he had a chip until he escaped and tried to kill Willow.
Audio problem: When Anya comes in from helping the potentials get in the car, she says that Molly is in the trunk. She then says something along the lines of "I'm surprised at how compact Molly is," as she walks away with Willow, but her mouth doesn't move.
Continuity mistake: When Willow is out with Kennedy, Willow tells her that she doesn't get why she likes her. As she says "I still don't get it", she pulls her hair back over her left ear. Then when she says "Why you like me" in the following shot, her hair is hanging back down over her left ear again.
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 ★