I recently watched Elysium. I'm a big fan of Jodie Foster and she's easily my favourite actress (see Silence Of The Lambs, The Accused and Taxi Driver), so I was surprised how bad her performance was in this movie.
Another example is Daniel Day Lewis in Nine. Again, he's my favourite ever actor (his performance in There Will Be Blood is probably the greatest ever in my opinion) but he was just was not right for that role at all.
What other great actors and actresses have you seen deliver a performance far below their talents on one occasion or more?