Out Of Mind, Out Of Sight - S1-E11
Revealing mistake: You get a pretty fast but noticeable glimpse at the stunt-woman's face when Harmony is pushed down the stairs, and it's obviously not the actress.
Welcome To The Hellmouth (1) - S1-E1
Revealing mistake: When Buffy sneaks into the locker room and checks out the dead body, you can see part of the outline around the prosthetic vampire bite on his neck (it isn't quite blended in properly), revealing it's an appliance.
Once More, With Feeling - S6-E7
Revealing mistake: When Buffy is singing in the Bronze with Anya and Tara as backup you can see Tara bump into the pillar and then dust herself off. (00:41:45)
Dead Things - S6-E13
Revealing mistake: In "Dead Things," after Buffy and Spike sleep together, when Spike asks, "Are we having a conversation?" and Buffy says, "What? No?" There is a side shot of her. If you look closely at her back (pausing helps), you can just barely see Buffy's skin colored tube top. Trust me, it's there. The time code is 1:22.
Revealing mistake: In the scene when Oz stops the female wolf from attacking Willow, and they both change and Oz tears her neck out, if you look closely, you can see the actor's real skin at the neck.
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.