I remember this now! I forget most of the details, but the girl's parents (or whoever) become convinced that she's evil. They plan a trip to a lake, telling her that it will be fun and as the episode ends, she looks into the mirror and sees water rising up over her head and in a sad voice repeats "Goodbye, goodbye..."
It was the second episode of the third season of Tale from the Darkside, called
I Can't Help Saying Goodbye