Boozing

The Buffyverse clearly states that alcohol = bad. Almost every time someone drinks any, something bad happens.

Teacher's Pet

When the new susbstitute/very attractive teacher asks Xander to come over to her place to help her out on a 'project' he ends up having drugged drinks and passes out. When he comes to, he's trapped in a cage and his teacher, actually a giant praying mantis, is planning on mating with him and then chopping his head off.

Reptile Boy

Upset by Angel, Buffy attends a frat party with Cordelia where the guys give them alcohol. The girls pass out and wake up chained up in the basement, ready to be sacrifed to a giant snake demon.

Beer Bad

After Parker dumps her, Buffy drowns her sorrow in beer with some college guys and the five all revert to prehistoric men and women.

Something Blue

When Oz leaves her, Willow is distressed and gets drunk. She ends up doing magic that goes wrong - causing Buffy and Spike to fall in love, Xander and Anya be chased by demons, Giles go blind and Willow herself almost become a vengeance demon.