Worst movie starring future Oscar winner

I don't mean a minor, blink-and-you'll-miss-it moment such as Marisa Tomei in The Toxic Avenger.   I mean  an actor/actress who was one of the top-billed cast members in an absolutely terrible movie who later went on to win an Oscar.   It can't be a case of an established star in a slump, it has to be before the person became a star in his/her own right.  I haven't seen it, but, from what I understand, B*A*P*S starring Halle Berry would likely qualify.