It was the live performance heard around the world – and landed with a thud. As part of the Kennedy Center Honors in 2006, Simpson saluted Dolly Parton but kept flubbing the words to "9 to 5." That didn’t prevent the pop star from going full country two years later with Do You Know, a slick Nashville outing that debuted at Number One on the country charts. (And to prove there were no hard feelings, Parton sang harmony on the title track.) – J.R.