In recent years, I think the biggest shock was Crash beating Brokeback Mountain in 2005, as the latter had won just about every other Best Picture award all award season till that point.
It was more disappointing than surprising, and it certainly was not shocking. For Hollywood to even recognise a film of such content was a step forward, but for it to win was just too much as far as they were concerned.
Crash was one of those awful films that likes to think it's dealing with political and social problems without actually scratching the surface or really offending anyone. Hollywood loves that shit. It was an acceptable substitute in an otherwise weak year in their eyes.