"I opened the envelope and it said, 'Emma Stone, La La Land,' " said Warren Beatty

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It was the biggest upset in Oscars history — Warren Beatty and Faye Dunaway mistakenly called La La Land as the winner of the 2017 Academy Award for Best Picture Sunday night. As the cast and creators filed on, the celebration was abruptly halted.

La La Land producer Jordan Horowitz stepped to the microphone and said, “There’s been a mistake. Moonlight won.”

“It’s not a joke,” he said. “This is not a joke, come up here.”

As the Moonlight cast walked up, Beatty, 79, would not leave the stage until he explained what had happened.

“I opened the envelope and it said, ‘Emma Stone, La La Land.’ That’s why I took such a long look at Faye and at you. I wasn’t trying to be funny,” he said.

Somehow, one of the two printed envelopes for Best Actress apparently wound up in Best Picture presenter Beatty’s hands. Confused, Beatty looked to his co-presenter Dunaway, 76, for help, and she ended up announcing La La Land. The musical’s cast and crew were in the middle of their acceptance speeches when accountants from PricewaterhouseCoopers took the stage to reveal the mistake — the biggest error at the live telecast in recent history.

Faye Dunaway holding an apparent Best Actress envelope from which she incorrectly announced La La Land as Best Picture at the 2017 Academy Awards.
| Credit: Eddy Chen/ABC via Getty Images

How Beatty got the wrong envelope remains to be seen. Best actress winner Emma Stone insisted she had her winning card, though there are copies of the cards and envelopes for each category.

PricewaterhouseCoopers, which handles the Academy Awards balloting and ensure it remains secret, has not commented. A spokeswoman told PEOPLE in the wee hours of Monday that the consulting firm was preparing a statement.

Beatty told reporters afterwards that he didn’t know how he got the wrong envelope, saying, “I’ve asked and I haven’t got an answer.”

La La Land producer Fred Berger was the first to break the news live. Suddenly, in the middle of his speech, Berger remarked, “We lost by the way but you know.”

Beatty, who presented the award with his Bonnie and Clyde costar Dunaway, said he was given the wrong envelope and was confused, apologizing.

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In the moment, Beatty looked confused at the envelope and hesitated, before Dunaway peeked over at the card and announced La La Land.

A production source was overheard backstage at the show, asserting, “It was NOT Warren’s fault!” The whole scenario was full-on chaos, with things like, “Oh my f—- God!” and “He’s got the wrong envelope!” being yelled.

Host Jimmy Kimmel said, “Guys, this is very unfortunate what happened. Personally, I blame Steve Harvey for this. I would like to see you get an Oscar anyway, why can’t we just give out a whole bunch of them?”

The Moonlight cast and crew slowly made their way to the stage as the La La Land cast dispersed, with the actual winning film’s director Barry Jenkins heading to the microphone first.

“Very clearly, very clearly in my dreams this could not be true but hell with my dreams, I’m done with it because this is true,” said Jenkins. “Oh my goodness. And I have to say it is true. It’s not fake. We’ve been on the road with these guys for so long – My love to La La Land. My love to everybody. Man.”

After the Moonlight cast celebrated their win, Kimmel again addressed the shocked crowd at the Dolby Theatre, “Well I don’t know what happened. I blame myself for this. Let’s remember, it’s just an awards show.”

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“I mean, we hate to see people disappointed, but the good news is we got to see some extra speeches,” he asserted. “We have some great movies. I knew I would screw this show up, I really did. Thank you for watching. I’m back to work tomorrow night on my regular show. I promise I’ll never come back. Good night!”

“I noticed the commotion that was happening, and I thought something strange had occurred,” Jenkins said in the press room. “I was speechless. It made a very special feeling even more special, but not in a way I expected.”

He added, “The last 20 minutes of my life have been insane. I don’t think my life could be changed any more dramatically!”

There has yet to be a formal explanation issued of how Beatty got the wrong envelope, but a rep for Best Actress presenter Leonardo DiCaprio told PEOPLE that “He gave the card directly to Emma and said ‘You are going to want to keep this.’ ”

Stone confirmed that she had her envelope, telling reporters, “I also was holding my Best Actress in a Leading Role card the whole time. I’m not sure what happened, but I really wanted to talk to you guys first. I think everyone’s in a state of confusion still. Excitement but confusion. I think everyone is just so excited for Moonlight. It’s such an incredible film.”

• Check out PEOPLE’s full 2017 Academy Awards coverage and complete winners list!

Two PwC staffers previously told MarketWatch that for security, there are “two briefcases, that are identical, and we have two entire sets of winning envelopes.”

Jenkins further told reporters, “I will say, I saw two cards, and so things just happen. I wanted to see the card, to see the card, and Warren refused to show the card to anybody before he showed it to me. And so he did. He came upstairs and he walked over to me, and he showed the card.”

“Everyone was asking, can I see the card? And he was like, ‘No, Barry Jenkins has to see the card, I need him to know,'” Jenkins said. “And I felt better about what had happened. I will say, the folks from La La Land were so gracious. I can’t imagine being in their position and having to do that.”

Despite the show’s unexpected ending, Dunaway appeared to be in good spirits as she arrived to the Vanity Fair party. She ate an In-N-Out burger and mingled with guests late into the night. “She was having the time of her life,” says a source.