Welcome To The Hellmouth (1) - S1-E1
Continuity mistake: This is hard to describe. When Buffy and Joyce are talking in her room, Joyce keeps pulling pillows off Buffy's bed and re-arranging them. However, at one point, when the shot cuts back to her, she's holding a different pillow than in her last shot. This technically wouldn't be a mistake since she's off camera except in the shots facing Buffy, there's a mirror behind her with her bed in view, so we should have Joyce put down the first pillow and pick up the second one... and we don't.