Continuity mistake: In the episode (series six) where Buffy thinks she killed Warren's ex-girlfriend, when she's at the police station, in the over the shoulder shot of Desk Sergeant, he is holding the phone between his shoulder and ear. But in the front shot of him the phone has jumped to his hand.
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.