Beneath You - S7-E2
Revealing mistake: When Buffy throws Spike into the benches at the church you'll notice that as he is about to hit the floor there is a blue mat where he lands. (00:36:15)
Revealing mistake: When Anya sings her song about how she will become Mrs. Harris, she spins Xander's chair around after putting a blanket on him. When she is also sitting on the chair, and you see an overhead shot of them both, you can see a section of the stage floor rotating beneath them keeping the chair spinning.
Revealing mistake: As Buffy throws Spike into a wall after he bites Andrew, Spike's stuntman is visible in-shot. His hair is slightly different to James Marsters'.
Revealing mistake: When the ubervamp is at the door of the Summers' home, you can see a black string attached to the inside of the door.
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.
Lies My Parents Told Me - S7-E17
Revealing mistake: When Spike tells his mother that he's become a vampire, she asks him if he's drunk. He responds with "Little bit." Just after he says this, someone can be heard coughing off-camera. It could be his mother because she isn't supposed to be well, but it seems odd that the camera wouldn't be on her during this. And a cough wouldn't really be necessary for the scene.
Dirty Girls - S7-E18
Revealing mistake: In the opening scene, where Caleb picks up the potential slayer in his truck, there is a brief shot of the two from the hood that pans slightly to the right. In doing so it reveals the edge of a screen showing a fence and trees passing by, revealing that the car is in fact stationary and in a studio. (00:02:40)
Revealing mistake: When we see Spike and Andrew on Spike's motorcycle talking about Onion Blossoms, a filming light is reflected in Andrew's helmet.
End Of Days - S7-E21
Revealing mistake: When Buffy finds Faith and the Potentials in the sewers she throws an Ubervamp across the room. When we see the Ubervamp from behind you can also see the actor's blue boxer shorts sticking out of the Ubervamp costume.
Revealing mistake: Early in the final fight, Vi makes a flying kick to knock an Ubervamp off the cliff. As she lands, she swings back and twists slightly, suspiciously like one would do if suspended on a wire. There is no way she could gain this motion by kicking straight off the vamp as shown. (00:29:00)
Revealing mistake: In the last episode, 'Chosen', Anya beheads two Ubervamps. The second time she swings the sword it very noticably doesn't go anywhere near the Ubervamp's neck, closer to its 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 ★