- The loss of Angel's soul and the return of Angelus during her 17th.
- Buffy being stripped of her Slayer powers in preparation for the Cruciamentum during her 18th.
- Rupert Giles being transmogrified into a Fyarl demon by Ethan Rayne during her 19th..
- Dawn Summers' discovery that she is the Key and subsequent identity crisis during her 20th.
- All of the participants of Buffy's birthday party, including anyone else who entered the house, being trapped inside the Summers residence by Dawn Summers' unwitting wish to Halfrek during her 21st.
- Her birthday is listed as 10/24/1980 on her high-school registration 
- In Nightmares her tombstone reads 1981 to 1997. Since this is Rupert Giles' nightmare, perhaps he doesn't know her actual year of birth? More likely it is just a continuity problem. 
Behind the scenes
- Whenever Buffy's birthday was celebrated on-screen, it was always during the twelfth or thirteenth episode of the season.