What happens when you say "don't smile"

Yashica Mat 124g: 1/60 @ f/11: T-max 100: 80 mm: Photogenic monolights: ©johannaworth2006

This is an outtake of one of my portrait assingments taken earlier this year. We were just using up film at the end of the shoot and she was tired of smiling. When I told her she didn't have to smile, this is what happened.