Three's Company - Season 6 Episode 12
Strangers in the Night

Jack falls in love with a beautiful woman and wants to serenade her. He goes to her apartment and sings at the window, but what he doesnt know is that he has serenaded an overweight girl. The overweight girls thinks that Jack likes her, but the beautiful woman trashes her in front of Jack. Jack decides to have a date with the overweight girl instead.