The Harsh Light Of Day - S4-E3
Other mistake: We've seen before that vampires are fine as long as they aren't in direct sunlight, yet in this episode Spike is in the shade when Buffy removes the ring and he still starts fizzling.
Other mistake: Just after Giles becomes completely blind, Spike gets up to get a book off of the mantle. When he does this you can, very briefly, see James Marsters reflected in the glass doors of the bookcase that was behind him, despite Spike being a vampire. (00:27:00)
Other mistake: In the scene where Spike escapes from a blind Giles, he pushes Giles out of the way and does not get a shock to the head from the chip in his head, which normally goes off with the most minor hint of violence.
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 ★