Factual error: When Angelus is torturing Giles in this episode he is seen with Giles' glasses, and he cleans them by breathing on the glass and wiping with a cloth. Vampires don't have breath, as Angel tells Xander in 'Prophecy Girl' from season one - even if he can force some air out of his body, it wouldn't be warm, moist air, which is needed for condensation to clean glasses with.
Continuity mistake: In "Becoming Part 2" At the end when Spike chokes Drusilla unconscious, he picks her up, one of her hands is resting on her stomach and the other is hanging to her side. It cuts to a shot of Spike looking at Angel and Buffy fighting then to Spike leaving with his back facing the camera. You can see that the hand resting on Drusilla's stomach moved to Spike's shoulder.
Other mistake: When Willow is attempting the spell in the hospital, the subtitles state that Xander is speaking a line of the incantation. It is actually Oz speaking - Xander is off rescuing Giles, as we see in the very next shot.
Revealing mistake: When Buffy and Angel are sword-fighting, the camera pans up and to the side, you can see David Boreanaz's stunt double. His hair line is farther back then David Boreanaz's.
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 ★