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.
Once More, With Feeling - S6-E7
Audio problem: When Tara sings to Willow and sings "nothing i can do", her lips don't move.
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.
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.
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.
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: 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.
Chosen answer: It is classic vampire mythology.
Twotall