Audio problem: When everyone is in the Bronze - just before the vamps show up - you hear the first line of a song being played, Ballad For Dead Friends by Dashboard Prophets, cut to outside and you see the vamps strolling along and killing the doorman, then cut back into the Bronze and the same line plays again. This is a total error in cutting the show, it is stated there is no live band but still the same line plays twice when that is not how the song goes.
Audio problem: At the beginning of the episode at the Bronze, when Buffy gets up to go and Willow tries to get Buffy to stay, Xander suggests they can dance. But his mouth movements don't match the words when he says "Eh, yeah, it's early..." (His head is turned away from camera, but you can see the side of his mouth, and he's saying something else).
Audio problem: When Xander is talking to Blaine in the cell, Blaine is explaining what Miss French/She-Mantis does. At one point Blaine says "She ties you up, " but his mouth doesn't match the words at all. His mouth and the words match for every other line, so I'm assuming they decided to add that line in post.
Audio problem: After the opening credits, we see a band playing in the Bronze. The lead singer's mouth doesn't seem to 100% match what he's saying. It's mostly right, but just seems a little "off." (Ex. There's a few very brief moments where individual syllables seem to not match, etc.) It's very clear he's simply lip-syncing to a pre-recorded track.
Audio problem: After Buffy catches the pig in the hallway, Principal Flutie tries to take him back. The pig squeals, making Flutie back up. However, it's painfully obvious that the pig isn't actually making the squeal and that it's just added in post. The pig looks completely calm and happy in Buffy's arms, it doesn't wriggle at all, and if you look closely, you can even see that its mouth doesn't move in the slightest. It's about as far away from squealing as possible.
Audio problem: When Buffy meets Angel, when she says the line "What I want is to be left alone!" and Angel replies "You really think that's an option anymore?" pay attention. Buffy's last few words don't quite match her lip movements, and Angel's lips don't seem to be moving at all for his line. The audio looping is a little sloppy. This is better seen in the brighter HD remaster, but is also noticeable in the original, darker standard definition master.