Not the biggest ever, but Kate Winslet winning for the Reader was a bit of a 'crap, she's been nominated 6 times for better films we better hurry and give it to her'. Her performance in Revolutionary Road in the same year was far superior. BUT she should have won for Eternal Sunshine of the Spotless Mind. She was amazing in that.
As she said in Extras, you have to player either a Nazi or a nun to get an oscar