Visible crew/equipment: Season 6, Episode 'Villains,' when Willow is chasing Warren through the forest, trees move out of her way to let her pass. When the trees behind her move back into place, in one shot you can see the rope they used to tug one of the trees down.
Revealing mistake: As Willow walks through the forest pursuing Warren, just after we first see Warren running, there's a shot of Willow walking into a bit of a clearing. Her shadow crosses a tree on the right (could be caused by the moon), but then a couple of seconds later another shadow is visible on a tree to the left - obviously an artificially lit forest. (00:33:15)
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.