Continuity mistake: In 'Gone', when invisible Buffy is typing on the social worker's keyboard the letter 'B' is pressed several times. When she prints what she's been typing seconds later it's revealed to be "All work and no play make Doris a dull girl", a phrase which contains no 'B's. (00:21:10)
Continuity mistake: In the kitchen scene Buffy takes the pizza out of the fridge and closes the door, but in the next shot the door is open and two shots later it's closed again.
Continuity mistake: After Doris from Social Services leaves Buffy's house and Spike leaves after her, he takes his lighter out of Buffy's pocket. But earlier in the episode when everyone was packing away all of Willow's magic stuff, Buffy finds Spike's lighter and puts it in one of the boxes.
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 ★