Audio problem: Near the end of the episode when Giles, Tara, Faith/Buffy and Willow arrive at the church and a police officer tries to stop them going inside Willow says, 'Oh no you don't understand (or words to that effect) but her mouth doesn't move at all, it was clearly dubbed.
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: Jenny prints off the translation for the Ritual of Restoration from her computer, then spots Angel sitting across the room from her. As she gets up and starts walking towards him the sound of the printer can be heard but the paper has already finished printing.
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.