I can think of two. One was Elizabeth Taylor, Best Actress for Butterfield 8. She was given this as the ultimate sympathy prize: she had faced down death and had had an operation. This is not to say that she isn't a fine actress, as you can see from some of her earlier films.
An equally great shame was Robert Redford beating Martin Scorsese in 1981 - Ordinary People over Raging Bull. It does seem tragic that Scorsese has never been given an Oscar.
And who was robbed?
As for the one that should have been given but wasn't, we start with Orson Welles for Citizen Kane, period.
Interview by Scott Hughes
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