If two characters making an acknowledged Buffy reference together is meant to be a relationship development point between them, it's probably a good idea to check if they haven't already done that two chapters beforehand (or, you know, remember you had them doing that.) Namely, if Stiles talks about the Hellmouth, and Derek doesn't question what that is; yes, yes both of them have clearly seen Buffy, why would they be surprised later in the story?
*headdesk* Oh, edit feature, I love thee. I can correct silly logic fails! Now, if only I could stop freaking making them, that'd be aces.