The Dick Tracy star was given the job of announcing the winner of the category alongside his Bonnie and Clyde co-star Faye Dunaway.
However, while La La Land had been expected to take home the gong, and Warren and Faye first stated that it had, it was then revealed that it was a mistake and Moonlight was actually the winner of the Best Picture prize.
Returning to the stage to explain what had happened later in the ceremony, Warren told the audience and host Jimmy Kimmel: "I want to tell you what happened. I opened the envelope and it said Emma Stone, La La Land," Warren said. "That's why I took such a long look at Faye and at you (Jimmy). I wasn't trying to be funny. Thank you very much. This is Moonlight, the best picture."
Emma also reacted to the mix-up as she was interviewed after winning her own Best Actress Oscar for Damien Chazelle's much-loved movie musical.
"I f**king love Moonlight. God, I love Moonlight so much!" Emma said in the press room after her win, according to USA Today. "(But) of course it was an amazing thing to hear La La Land."
Despite the mix-up, La La Land certainly didn't leave the Oscars empty-handed, taking home statuettes for Emma, director Damien, who, at 32, became the youngest winner in that category in Academy history, as well as winning Best Original Score, Best Original Song, Best Production Design, and Best Cinematography.
As well as winning Best Picture, Moonlight shone bright for Best Supporting Actor recipient Mahershala Ali, while the film's director Barry Jenkins and Tarell Alvin McCraney landed Best Adapted Screenplay.
Manchester by the Sea fared well too, with Casey Affleck taking home the Best Actor award and writer/director Kenneth Lonergan winning Best Original Screenplay.