Continuity mistake: When Willow hides under the table in the classroom after fleeing The Pack, we see that it is on legs and doesn't touch the floor. Yet when Xander comes in and scans the room, he sees the table but Willow's feet aren't visible, even though they should be judging by the height of the table from the floor.
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: 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.
Deliberate mistake: When Xander ducks under the caution tape to catch up to the group of students at the beginning, as the shot is about to cut, judging by his casual walking pace, he should maybe be 10 or so feet ahead of Buffy and Willow. When the camera cuts, he's suddenly at least 30 feet away. It's probably edited that way for time reasons (to quicken the pace of the scene rather than show him walking an extra 10 seconds), but still a minor mistake.
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.