Gimme a Break! - Season 3 Episode 15 Knock Three Times

Nell gets hypnotized to remember where she put Katie's ring for graduation, but she ends up remembering a whole lot of the girls' dirty laundry, since the doctor forgot to remove the hypnotic transe, and whenever she heard three knocks, she'd tell the absolute truth. In the end, she tells everyone that she is going to marry James, but tells him the truth that she can't marry him yet, that is until the kids are all grown up.