In a surprise upset that was somehow the last thing that the audience was expecting, Moonlight was awarded the Best Picture Oscar over La La Land, but not before a combination of errors led to presenters Warren Beatty and Faye Dunaway announcing that La La Land was really the winning film. In addition to Beatty and Dunaway, it's Leonardo DiCaprio that some people want to blame for the mixup — but what does Leonardo DiCaprio have to do with the Best Picture Oscars mixup? Well, as the 2016 Best Actor, he was onstage to present Emma Stone with her award for Best Actress in a Leading Role. Now, the reason that Beatty claimed that there was a mixup in the first place was because instead of the envelope for Best Picture, he was handed an envelope for... Actress in a Leading Role. Update: In case the evidence wasn't already clear enough that Leo had nothing to do with #envelopegate, the Oscars accountants from PricewaterhouseCoopers apologized for the flub hours after the ceremony, telling the Hollywood Reporter in a statement that they are investigating how the duplicate envelope ended up in Warren Beatty's hands.
Earlier: While walking offstage, though, it's DiCaprio who's holding the Best Actress envelope, not Stone, and it looks like he places the card back into the envelope and closes it, making some Twitter detectives think that it's possible that he's the one who set it down and managed to cause the mixup. But, According to EW writer Marc Snetiker's statement from Emma Stone, she had her winning envelope backstage with her "the whole time," so somehow there must have been some other kind of mistake — one that Leo had nothing to do with.
Despite the tweets blaming DiCaprio, if you look at some other shots, it does appear that Stone has an envelope by the time she heads all the way backstage, and DiCaprio's hands are empty.
And according to accounting firm Price Waterhouse Cooper, who tabulate the winners and print the cards announcement the selected films and performers, there are in fact two briefcases filled with envelopes, so there is a chance that somehow there were two Best Actress envelopes instead of one for Actress and one for Picture. A Medium interview with Brian Cullinan and Martha Ruiz about the process of how the presenters get envelopes seems to back that up.
I think it's pretty clear that there were several other factors that may have contributed to the shocking moment when Moonlight took Best Picture from La La Land. It is, however, slightly ironic that the man who fought so long and hard for his Oscar (never forget, he slept outside and ate bison liver) was unfairly dragged into one of the biggest screw-ups in Academy Awards history.