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.
Continuity mistake: When Xander thinks he hears someone at Miss French's house, she distracts him by caressing his chin. In the two shots facing Miss French, she's touching his chin with her LEFT hand but in the shot facing Xander she's touching his chin with her RIGHT hand.
Continuity mistake: When Buffy is telling Giles and Willow what she read about praying mantises in the book, she slams it shut and says "Ha!" The scene then cuts from a wide shot to a medium shot of Buffy, and the way she is holding the book changes slightly between cuts. (More specifically, her hand and the book rotate about 45° between cuts).
Continuity mistake: Near the beginning, Miss French picks up a little terrarium with the mantis in it, the position of the mantis changes instantly between cuts. First it's on the left side, then it's on the right.
Continuity mistake: At the Bronze in the beginning of the episode, two guys are talking about girls. At one point, one of them (the white guy) goes from facing forward to facing sideways at his friend between edits.
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.
Continuity mistake: In the beginning of the episode, when Buffy's teacher is looking at the slides, he goes from holding a slide with one hand to two hands instantly between edits.
Continuity mistake: In the beginning of the episode, Buffy's teacher turns on a light box and places a single slide on it. In the next shot, you can see the light box reflected in his eyeglasses, and in the reflection, you can see there are suddenly four or five slides on the light box. Then in the very next shot, there are suddenly NO SLIDES WHATSOEVER on the light box, and he places two down.
Continuity mistake: In the opening scene, the vampire makes Buffy back up into a pillar. In the first shot, Buffy is only inches away from the pillar. The camera cuts to the vampire for a second, then when it cuts back to Buffy, she's suddenly moved forward several feet and backs up into the pillar finally.
Continuity mistake: In the opening scene, when you first see the vampire, you see a girl behind him running from screen-right to screen-left. She's in a blue plaid shirt and jeans. The scene cuts to Buffy for a second, then cuts back to the vampire and you can see the same girl doing the exact same thing in the background... running from screen-right to screen-left. (It looks like they used two different takes of the same shot in the scene).
Continuity mistake: When Giles walks up to Buffy, Xander and Willow after The Body is found at the beginning of the episodes, watch the extras walking by in the background. They don't always match as the shot cuts back and forth between Giles and Buffy/Xander/Willow. Ex. When Buffy asks "Vampire?" nobody is walking behind her, but when it cuts to the opposite angle, suddenly two students are walking by from Buffy's direction that weren't there before. This sort of thing happens at least two or three times.
Continuity mistake: After the credits, when Buffy, Xander and Willow sit down to talk about what to do for the talent show, two people walk into the background behind Buffy and are standing there talking throughout the scene. At one point, it cuts to Xander for about one second when he says the line "Whatever happened to corporal punishment?", then cuts back to Buffy, and the two people behind her are suddenly gone. There's not enough time for them to have walked away between shots - they simply vanish.
Continuity mistake: When Snyder grabs Buffy after she breaks into the locker, when she turns around, he is holding her hand up by the wrist. The height he is holding her hand up changes a few times between shots. Sometimes it's up by her face, other times it's level with her neck, at one point it's down by her shoulder, etc. Obviously snippets of a few different takes were used, and he wasn't holding it up at exactly the same height each time.
Continuity mistake: When Willow is unsuccessfully trying to tutor Xander outside towards the beginning of the episode, after Willow warns Xander he might flunk math, he reaches up and massages his face with his hand. When he does this, he has a pen in his hand. When the camera cuts to the opposite angle, the pen vanishes. Then, when the scene cuts to a wideshot about 10 seconds later, suddenly his hand is in a completely different position between cuts.
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.
Continuity mistake: In the beginning, after the monk traps Moloch and closes the book, the positions of his hands gripping the sides of the book changes slightly between cuts. (Ex. His left hand is lower, then suddenly slightly higher between edits).
Welcome To The Hellmouth (1) - S1-E1
Continuity mistake: When Buffy meets Angel, Angel is walking down the alley and Buffy swings around a pole and kicks him from behind. Except you can tell in the first shot that Angel is too far ahead of her - she couldn't have reached him to kick him. She's only able to reach him through the power of editing, as the shot cuts, and he's suddenly closer to her. She also somehow flips backwards yet lands in front of him. The scene looks cool in motion, but makes absolutely no logistical sense if you watch closely.
Welcome To The Hellmouth (1) - S1-E1
Continuity mistake: In the beginning, when Darla "vamps out" and kills the boy, watch her shirt collar. In the first shot, her shirt collar is unbuttoned, but when she turns around to bite him, suddenly it's buttoned closed.
Welcome To The Hellmouth (1) - S1-E1
Continuity mistake: In the beginning, when Darla and the boy break into the school, the boy breaks through a window, reaches in, and unlocks the window from inside. When he does this, as the shot changes, the broken glass around the window changes slightly between cuts. (Noticeably, the glass wedged between his hand and the window vanishes.) Additionally, in the first shot, there's a microscope right next to the window, about an inch away. In the next shot, it seems to have moved about four inches to the left.
Continuity mistake: When the zookeeper catches Buffy and Willow trying to sneak into the hyena exhibit in the beginning, when Buffy and Willow turn around, they get close to each other but there's still at least 6-8 inches of space between them. When the camera cuts, though, they are suddenly pressed literally right up against each other, to the point they are borderline huddling.
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.