Continuity mistake: When Oz turns back into a werewolf, he chases after Tera. She hits him with a chair and he falls to the ground and passes out. We then see The Initiative show up and learn that Riley has shot Oz with a dart (it's sticking out of Oz's side). But two shots earlier the dart wasn't there.
Continuity mistake: When Oz shows up at Buffy and Willow's dormitory, Willow has a lump or a pimple above the left corner of her mouth. It disappears later in the episode when she's talking to Buffy about her feelings for Tara and it returns at the end when she and Oz are in his van.
Continuity mistake: Buffy gets a massive scar on her head from fighting with Adam. This scar changes size at least twice throughout the rest of the episode.
Continuity mistake: We see Xander lying in bed with his chest and stomach uncovered. But when Anya comes in and starts talking to him about checking in at the unemployment office, his bed sheets are suddenly pulled up covering his chest and stomach.
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 ★