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: 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.