Welcome To The Hellmouth (1) - S1-E1
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.
Audio problem: When Buffy kills the vampires dragging Xander at the beginning, we hear two impact sounds but don't hear either of them turn to dust. They were only a few feet away and there was no other sound to cover it up. (00:01:00)
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: 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: 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).
Nightmares - S1-E10
Audio problem: When the Scooby gang goes back to the hospital to try to wake up Billy, they talk to the doctor, who mutters "My hands." However, his words don't match his lip movements in the slightest. He seems to be muttering something else entirely.
Out Of Mind, Out Of Sight - S1-E11
Audio problem: When Cordelia is tied to the chair at the end of the episode from a side view you can see her mouth 'Marcie' but can't hear it and in the front shot she says 'Marcie' again.
Out Of Mind, Out Of Sight - S1-E11
Audio problem: When the invisible girl writes "Listen" on the chalkboard, the chalk sound effects don't match the writing for the last few strokes. (They're about a half-second too "early.").
Out Of Mind, Out Of Sight - S1-E11
Audio problem: When Cordy and Buffy are talking and Cordy says "Well it beats being alone all by yourself, " pay attention. It's clear that part of the line was dubbed in, as there is a very hard cut between the words "Well" and "It."
Out Of Mind, Out Of Sight - S1-E11
Audio problem: When Cordelia wins May Queen, the clapping sound effect doesn't match the students clapping in the slightest. It's far too loud considering it's a small group outside, and continues on for about two seconds after the students clapping.
Audio problem: When Willow and Jenny jump in Cordy's car, Cordy mentions that she saw "these things coming at me!" However, her mouth isn't moving for the last few words.
Audio problem: When Giles and Miss Calender are walking through the halls at the beginning of the episode there's a kid at the vending machine, we hear the coins rattling but he never puts money into the slot. In fact, the money doesn't go anywhere near the slot.
Audio problem: When Jenny and Giles are talking and Jenny is talking about the festival, they walk behind a drinks machine and then her lips don't match up with what she says.
Audio problem: When Xander, Willow and Ampata meet up at The Bronze, the song the band is playing is at the point of a guitar solo. If you watch Oz (who has already been established as 'lead guitarist') on stage in the background; he's blatantly NOT playing that solo. The hand movements don't match up.
Chosen answer: Angel is saying that he is still jealous and Buffy recognizes that this is his way of saying he wishes that they could be together. That is why they both smile.