I've tried so hard to reconcile what I find to be a fairly blatant flaw in my favorite show...
In the episode "Normal Again" Buffy claims to have been sent to an institution when she first told her parents she was a slayer (before the beginning of the series). However, if this is true, it's hard to imagine Joyce reacting in the way she did when Buffy 'came out' in "Becoming, Part 2". One would imagine she would have more of a "Not this again" reaction as opposed to the utter shock and denial she showed.
I'd love some hard facts on this, but I'll gladly accept some good fanwanking theories!