Continuity mistake: The security tag on Buffy's new jacket shifts when Buffy first sees it. (00:12:50)
Visible crew/equipment: When the Scoobies are attempting to leave Buffy's house by using magic, we see a close up shot of the house's front door knob. None of the Scoobies are reflected in the door knob even though they're standing right in front of it. There is also a fairly large black object reflected in it. A film crew perhaps?
Other mistake: As Spike follows Buffy into the hallway by her front door we see Spike's reflection in the mirror. Vampires do not have reflections, so Spike should not have been visible.
Continuity mistake: The wreath on the front door of the Summer's house disappears just after Tara tells Willow she will be doing the spell. It reappears in the next shot from outside the house.
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 ★