For this year's Costume Institute Gala
, 34-year-old Mendes says she wasn't feeling 100 percent, even with her gorgeous dress (designed by her date, Calvin Klein designer Francisco Costa). "I felt terrible that I wasn't giving it my best and worried that I would let Francisco down. I came home early and thought, 'Well, scratch that one; we'll get 'em next time," she says. "Yet, the next day when I saw the pictures, I was stunned because they said I really rocked it. I think I did live up to the dress, after all."