Storyteller - S7-E16
Revealing mistake: When Buffy, Spike, Andrew and Robin go to the high school and find it decimated, Buffy is thrown to the ground and attacked with a hockey stick. On the second swing, a sound effect is heard and Buffy falls over. But the hockey stick never makes contact with her.
Storyteller - S7-E16
Continuity mistake: In the scene where Xander and Anya have had their "one more time" and are lying in the cellar bed talking, the shot is on Xander and he is speaking. When he finishes speaking Anya starts, but the camera stays focussed on him, however, you can see part of Anya's face in the shot and her mouth is not moving despite the fact she is talking.
Storyteller - S7-E16
Visible crew/equipment: During Andrew's fantasy of him, Warren and Jonathan being Gods, something dark is reflected in of the piles of gold. Possibly a camera or camera crew.
Storyteller - S7-E16
Audio problem: After Anya and Xander sleep together again, Anya says "I think, maybe we're really over. Which - it's good, right? I mean, now we can move on." When she says "Which - it's good", her lips don't match what she's saying.
Storyteller - S7-E16
Continuity mistake: There is a shot where a girl is turning invisible, and when the camera switches to Buffy slapping her, she is fully visible, until it turns back and she is slowly becoming visible again.
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 ★