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.
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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.
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.