Revealing mistake: In the opening of the season 6 episode "Two To Go", Willow has gone crazy and is trying to kill the two remaining members of the Trio, so she is pulling the brick wall of the jail apart brick by brick. The problem is that if you look carefully, they are obviously not bricks being pulled away; they look more like plastic or plaster.
Continuity mistake: When Willow is outside the jail just before she starts pulling bricks out, there are two policemen walking past the open door, but a split-second later as the shot moves out to look up at the window, they disappear.
Continuity mistake: Where Willow is pulling the bricks out of the jail near the beginning of the episode. When she first pulls the bricks out of the arch thing they are from a different area than when you see the arch from the inside of the jail, even allowing it to be 'inside out' (as seen from outside and then the inside).
Visible crew/equipment: After Anya teleports to the jail leaving Buffy and Xander in the forest, the shadow of cameraman walking backwards can be seen as Buffy and Xander make their way to Xander's car. Look closely, the shadow is visible and Buffy tells Xander not to be too sure about Anya being able to handle Willow tonight.
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.