The Inglourious Basterds star was widely expected to receive a Best Actor nod for his portrayal of Brandon Sullivan, but he was edged out by Brad Pitt, George Clooney, Gary Oldman, Demian Bichir and eventual winner Jean Dujardin.
And Fassbender sheepishly confesses he was blindsided by the snub.
He tells GQ magazine, "At the beginning people (say), 'You're going to be going to the Oscars,' and you're like, 'Whatever, doesn't matter, don't think so.' But after a while it does penetrate. After a while you're like, 'Anyway, so I'm going to the Oscars...' And you start to believe it. And I did. I thought I was going. And then I found out I wasn't and I was upset. I was very upset by it. The first reaction was, 'What the f**k?' It's a vanity thing. It does become important to you. And it shouldn't."
And the British actor is certain his sexually-explicit love scenes and full-frontal nude shots are too blame - because prudish Academy voters still aren't ready for such a controversial film.
He's also convinced a scene in which his character urinates was the difference between him attending the Academy Awards and being frozen out: "That peeing cost me an Oscar.
"After a certain point you worry that it kind of detracts from the movie. But there's nothing I can do. I just have to laugh it off. I can. Pretty much."