Other mistake: In the episode 'Anne' Kenn tells Buffy that time in his hell dimension passes more quickly, that one day on Earth is the same as 100 years in his dimension. This means time goes roughly 36,000 times quicker. When Lily falls into the portal, it's at least 10 seconds until Buffy jumps in after her, but Lily is still just sitting there looking stunned. If times moves that much faster, 10 seconds would become over 4 days. Would Lily still have been just sitting there?
Other mistake: When Angel fights with Buffy in the forest you can see that he hasn't got shoes on, just socks. Later, when Buffy goes back to the mansion and Angel is chained up, he's wearing shoes. How did he manage to put them on, if he was in chains?
Lover's Walk - S3-E8
Other mistake: In the final scene, where Spike drives down the road while singing 'My Way' he turns the steering wheel significantly, but in the external shots the car is travelling down a straight road, with no bends. He is turning it sharply to the right as the camera cuts to outside, but the car continues straight ahead.
Other mistake: When the detective is interviewing the woman in the alley he says "So you heard the man scream about what time?", but Finch didn't scream as he died.
Other mistake: In "Doppelgangland", Vampire Willow struggles to remember Cordelia's name. However, it is still Willow, she knew Cordelia from being human (we see this in The Wish as well).
Other mistake: When we first see one of the Hellhounds at the clothes store that Cordelia works at, it jumps through the window, smashing the glass, and then attacks a customer who's wearing a tuxedo. But the Hellhound attacks the customer several seconds after it smashes through the window, giving the customer a chance to escape. But the customer just stands there unaware of the Hellhound, having not heard the window being smashed.
Other mistake: At the end the library that was rigged with explosives blows up. But the school's main entrance and the clock tower blow up as well, even though they are not rigged with explosives.
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 ★