Visible crew/equipment: When the knights start advancing in the bottom right corner of the screen you can see a crew member wearing a red plaid shirt just standing there.
Visible crew/equipment: When Ben arrives in his car and discovers the Knights of Byzantium, the shadow of a camera on a crane can be seen in the bottom left corner of the screen as he pulls up.
Continuity mistake: The shape of the hole in the aluminium foil in the Winnebago's windshield changes shape several times throughout the course of the episode.
Continuity mistake: At the beginning of the episode when Glory gets hit by a truck, she lands on the roof of a car twice.
Revealing mistake: Watch the shot of Glory as she is hit by the truck at the beginning of the episode (easier to see in slow-mo/frame-by-frame). She isn't hit by the truck at all. It goes right past her. A simple optical illusion used in filmmaking for decades.
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 ★