Revealing mistake: As Willow is raising the satanic temple from the earth, we can see the support cables holding the temple upright from behind. This is plainly visible in the bottom right hand corner, complete with bright orange clasps on the temple itself.
Continuity mistake: During the opening of the season 6 episode "Grave," Willow's nose starts bleeding and she gets blood on her upper lip. Over the next several shots the blood consistently keeps appearing and disappearing.
Visible crew/equipment: When Xander is leaning over the hole in the cemetery into which Buffy and Dawn have fallen, he causes the edge of the hole to collapse. Seen from below, everything is fine, but the shot from above Xander shows a hand grabbing hold of the back of his trousers and his briefs are visible.
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.