Leading Performances that were actually Supporting.

Dustin Hoffman in Midnight Cowboy. Mike Nichols even tried to persuade him to not accept it as The Graduate established him as a leading man.

Maximilian Schell in Judgement at Numberg. It's really an ensemble picture with no true lead so him having that little screentime yet not only getting a nom but winning? At least I can kinda see Spencer Tracy's nom here since he at least has the most screentime (I know that's not a good way to judge a lead performance)

Faye Dunaway, Peter Finch and William Holden in Network. Again, much like Judgement at Numberg, it's an ensemble. Going back to the screentime debate, Dunaway actually has more screentime than Talia Shire in Rocky, also nominated in that category, but I can buy Shire's placement since Rocky is about her just as it is about Rocky.

And before anyone puts Olivia Colman in The Favourite, you could argue all 3 women were leads but Colman's character remains the focal point of the entire film so I can see that placement.