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.
Continuity mistake: When Timothy shows Buffy how to use the grill, there is a wood-handled grill press, and another tool, right in front of the button panel. However, after he says, "It eliminates variation," in the close-up as he presses the button, both tools as well as the transparent plastic glove he wears have all vanished. (Another mistake notes a different close-up with a disappearing glove.)
Continuity mistake: When Buffy is assigned to the grill, as Timothy explains the process he's wearing a short-sleeved orange/white stripe uniform. In the close-up, when he places the second patty on the grill, Timothy is wearing a 3/4 sleeve, dark blue shirt.
Visible crew/equipment: There's a boom mic visible when Cordelia is talking to Harmony and the other Cordettes. It's the first scene in the alternate universe and they're standing inside a classroom of Sunnydale High (Cordy says something about going to the Bronze).
Revealing mistake: Right at the beginning of the episode just before we see Warren squirt Jonathan with water, we see Buffy on one of their security cameras. The footage of her is pre-recorded as the progress bar for QuickTime Media Player is visible at the bottom of the screen. The window was moved down to hide this fact, but not far down enough to hide it completely.
Continuity mistake: After Buffy decides not to take the antidote to the demon's venom, we cut to a scene at the mental institution. The doctor talks to her about ridding her mind of the things that support her "hallucinations". As he talks to her about this, the position of his hands change from pressed together in front of him to in his pockets between shots.
Continuity mistake: When the gang are fighting Glory's minions, one of them knocks Giles to the ground and then tries to choke him. As Giles hits the ground he lands facing downwards so we can't see his face. But when the minion is chocking him in the shot immediately afterward, he is suddenly facing upwards.
Revealing mistake: Near the end when the good Willow says goodbye to the evil Willow and hugs her, there is a shot of the back of the good Willow's head and the evil Willow's face. You can tell that the good Willow is a body double wearing a wig - there's something circular near her scalp, possibly an elastic band.
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.
Revealing mistake: When Jonathan assists Buffy at the beginning of the episode, he enters through the glass ceiling of the mausoleum, which shatters into squares. Although the audio is the sound of glass shattering, the pieces of "glass" flutter down, revealing that the squares are paper - simply for the safety of the actor during the stunt.