Well, that's an Oscars for the history books. Warren Beatty and Faye Dunaway announce La La Land for Best Picture — but it's a mistake. The envelope apparently contains the winner for Best Actress (Emma Stone, for La La Land), instead of Best Picture. Watching the video with the benefit of hindsight, it's clear that Beatty and Dunaway are confused about what they see on the announcement card in the envelope, but in the moment, decide to push forward anyway.
Moments later, with La La Land's cast and team on stage and thinking they'd won Best Picture, producer Jordan Horowitz steps to the microphone and holds up the actual winner of Best Picture, telling the world, "This is not a joke, Moonlight has won best picture."