This slip-up is in the season 5 ep. "Crush." When Dawn goes to Spike's crypt and he comes out of his little hole, you see that he has a cigarette behind his ear. As he is talking to Dawn you see him pick up the pack and he takes out one, and also the cigarette is gone behind his ear. I'm not sure but it might reappear; you'll just have to check it out. It's quite obvious.