I Only Have Eyes For You - S2-E19
Audio problem: When Giles looks through the window of a door at the high school and sees the janitor yelling at the teacher, the janitor's lips don't match with what he is yelling.
Audio problem: The drummer in Oz's band's movements don't match the song that is heard when we first see them performing at Buffy's house.
Audio problem: While Faith is stabbing the demon in his apartment, we hear the sound of her knife going through, and him yelling in pain. However, the last sound of her stabbing him comes a little late: The knife isn't even touching him.
Audio problem: When Joyce is about to leave for the pre-show, we hear Xander say "Have fun, but not too much fun", but his lips never move.
Audio problem: When Joyce comes downstairs and comments on the breakfast Buffy made, Dawn says 'Oh, Buffy helped,' but her mouth doesn't match what she's saying.
Audio problem: At the start of the episode when Buffy and Dawn are with Joyce in the hospital room, Dawn starts eating jelly, and when Joyce starts speaking you can see in the left hand corner of the screen Dawn talking, but no words are coming out.
I Was Made To Love You - S5-E15
Audio problem: When Spike arrives at the magic shop with a burning blanket over his head, he throws it to the ground and stomps on it, yelling, "Fire! Fire!" As he yells "Fire! Fire!" however, his lips aren't moving.
Once More, With Feeling - S6-E7
Audio problem: In the episode 'Once More, With Feeling', when Anya sings her first few lines of the song 'I'll Never Tell', you can still hear Xander talking about omelettes in the background, but for the last few lines he speaks (from 'I could do an omelette' onwards) you can see over her shoulder that his voice is dubbed. (00:13:10)
Once More, With Feeling - S6-E7
Audio problem: When Tara sings to Willow and sings "nothing i can do", her lips don't move.
Once More, With Feeling - S6-E7
Audio problem: During Buffy's first song she does a spinning kick to a vampire's face as she sings, "I've been making shows of trading blows, just hoping no one knows." But as she spins, her lips are not moving.
Once More, With Feeling - S6-E7
Audio problem: In the final song "Where Do We Go From Here", you can hear Spike still singing even after he has apparently stopped. He and Buffy continue singing until after the line 'When do the trumpets cheer'. It is noticeable because James Marsters sings the word 'cheer' slightly out of sync with everyone else.
Audio problem: When Warren, Jonathan and Andrew break into the museum to get the diamond, Warren makes a hole in the glass, then pushes it through, but when it lands, it sounds like plastic, not glass.
As You Were - S6-E15
Audio problem: When Buffy has just killed the Suvolte demon and Riley is found out to have not told her it was a tracking only mission Sam says something like 'If we weren't under time constraints, I'd rip you a new one.' Riley then says 'Stand down soldier.' Buffy asks Sam, 'Is he your boss?' And Sam replies 'He wishes. We better regroup.' But her lips to not move when she says they have to regroup. You can see her mouth on screen and she is just smiling, not moving her lips at all.
As You Were - S6-E15
Audio problem: When Buffy and Riley are fighting the Suvolte demon at the dam, Buffy calls out Riley's name at some point during the fight. Her lips don't move.
Audio problem: When Buffy is about to close the door as she goes into the bathroom, it actually never closes. But we still hear the sound of the door closing. (00:15:20)
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 ★