Revealing mistake: When Buffy is invisible and in the social office in "Gone", she is searching for the summer files, and you can see a thumb picking them up. (00:20:40)
Revealing mistake: When Buffy is at the desk of the social assistant you can see the white wire at the left of the keyboard. The white wire controls the fake keyboard: you can see the wire moving. (00:20:50)
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.