When it came time to announce the Best Picture award, Faye Dunaway and Warren Beatty came out to mark the 50th anniversary of Bonnie and Clyde. Beatty read the contents of the envelope, then stalled for a bit. Dunaway thought he was fooling around. He handed it to her and she read "La La Land."
The La La Land people gave their acceptance speeches, and then someone came out and said there had been a mistake. Moonlight had won Best Picture. The PricewaterhouseCoopers representative had given Beatty the Best Actress envelope, which went to Emma Stone for La La Land.
The PricewaterhouseCoopers representative was later shown to have been texting backstage immediately before giving Beatty the wrong envelope. Both representatives have been banned from future ceremonies. Both of them failed to act when the wrong winner was announced.
This had never happened before.