Going to see the dentist
 

I hope this one has not made the rounds here. It is an oldie but a goodie.

A woman who sees herself as not looking her age as well as told so by her girlfriends goes to see a new dentist since her old one retired. After she gets into the chair and he walks in he looks, moves and sounds familiar to her. She realizes he was a classmate of hers in high school, but he certainly hasn't aged well. She says to him "I think you were in my class in high school." He says to her "What did you teach?"