It was one of the most awkward moments in the history of the Oscars, of television, in entertainment, heck maybe in American history.

And somehow Warren Beatty, Hollywood鈥檚 ultimate smooth leading man, was at the centre of it.

The producers of 鈥淟a La Land鈥� were nearly done with their acceptance speeches for Best Picture, the Oscar broadcast鈥檚 credits sequence about to roll, when a stir of whispers began on stage. Moments later 鈥淟a La Land鈥� producer Jordan Horowitz returned to the microphone and said 鈥淢oonlight won Best Picture鈥� and insisting that 鈥渢his is not a joke.鈥�

The collective jaw of the crowd at the Dolby Theatre 鈥� and of America 鈥� remained dropped long after they became convinced it was no joke, but what academy historians later called an apparently unprecedented Oscar error.

Beatty returned to the mic and amid silence, explained that he had opened the envelope and he was confused when it read 鈥淓mma Stone, La La Land.鈥� He had shown it to co-presenter Faye Dunaway briefly, as though he wanted her to read it, which she did, apparently assuming the Emma Stone part was off but the 鈥淟a La鈥� part correct.

鈥淚t鈥檚 one of the strangest things that鈥檚 ever happened to me,鈥� Beatty said backstage. 鈥淭hank God there were two of us up there,鈥� Dunaway responded.

The actress then asked Beatty, 鈥淲ho else should I tell?鈥�

鈥淓verybody,鈥� he said.

At that point, a security guard tried to take the real envelope and Beatty said, 鈥淪ecurity is not getting this. I鈥檓 giving it to (Moonlight director) Barry Jenkins at a later time.鈥� Beatty also refused to show it to anyone else.

ABC News, tweeting about the ceremony broadcast on its network, said the envelope held by Beatty read, 鈥淎ctress in a leading role.鈥� A close-up photo of Beatty onstage verified that.

The mistake was being investigated by the academy, which had no immediate comment Sunday.

PwC, formerly known as Price Waterhouse Coopers, is the accounting firm that counts votes and provides winner envelopes for the Oscars and has done so for more than 80 years.

When the firm鈥檚 representatives realized the mistake, they raced onstage to right it, but too late, officials told The Associated Press.

The result was a bizarre scene with the entire cast of both movies standing together on stage exchanging sympathetic awkward stares and hugs.

The crowd had to rouse itself from its stunned stupor to try to give 鈥淢oonlight鈥� its just due for winning the big award. And as the credits rolled, usually bringing sweet relief after a long night, people appeared hesitant to leave in their disbelief.

鈥淚t made a very special feeling even more special, but not in the way I expected,鈥� a bemused Jenkins, co-writer and director of 鈥淢oonlight,鈥� said backstage.

鈥淭he folks at 鈥楲a La Land鈥� were so gracious,鈥� he added. 鈥淚 can鈥檛 imagine being in their position and having to do that.鈥�

Filmmaker Kenneth Lonergan, who won a best screenplay award for 鈥淢anchester by the Sea,鈥� cracked wise backstage: 鈥淚t turns out that we actually won best picture, which we鈥檙e really happy about,鈥� he said.

The ceremony鈥檚 chaotic scene immediately raced to the top of all-time Oscar moments, far more stunning than the nude man who raced across the stage in 1974 as part of the 鈥渟treaking鈥� fad.

And it immediately evoked Steve Harvey鈥檚 recent gaffe of naming the wrong Miss Universe winner.

The Miss Universe Twitter account quickly let the Oscars know they felt their pain.

鈥淗ave your people call our people, we know what to do,鈥� the tweet read.

That was the beginning of a Tweet pile-on that may be the biggest of all time, with many declaring that 鈥淟a La Land鈥� won the popular vote while 鈥淢oonlight鈥� won the electoral college.

But this stage, and this audience, were far, far bigger and are likely to last far, far longer in collective memory.

鈥榃e thought it was a joke鈥�: Canadian winner for Arrival

Montreal sound editor Sylvain Bellemare has an epic memory to accompany his Oscar win for 鈥淎rrival.鈥�

The Canadian was still in the Dolby Theatre on Sunday night as Warren Beatty and Faye Dunaway wrongly announced 鈥淟a La Land鈥� as best picture. Bellemare said it was 鈥渁ll very embarrassing鈥� as producers scrambled on stage to reveal that 鈥淢oonlight鈥� was the true winner.

鈥淚t was astonishing for like 15, 20 seconds. I think we thought it was a joke and no, it was real,鈥� Bellemare said by phone from Los Angeles after the show had ended.

鈥淏ut yet people were really happy I think because 鈥楳oonlight鈥� is a fantastic film that deserves so much,鈥� he added.

鈥溾�楲a La Land鈥� is a great film, too, but it already has a lot of love and box office since it started and having 鈥楳oonlight鈥� triumph, it鈥檚 perfect.鈥�

The accounting firm PwC, which oversees the counting of the Oscar ballots, said early Monday that Beatty and Dunaway had been given the wrong envelope.

Bellemare said 鈥渁t the end, things went as they were supposed to be.

鈥淚t was a weird feeling鈥�. It was something awkward but those things happen and at the end, at least we got the real winner onstage.鈥�

Bellemare won the Oscar for best sound editor in a field that also included teams from 鈥淒eepwater Horizon,鈥� 鈥淗acksaw Ridge,鈥� 鈥淟a La Land鈥� and 鈥淪ully.鈥�

鈥淚 had a good handshake by Ryan Gosling when I went to the stage,鈥� he said, adding that his thoughts were with 鈥淎rrival鈥� director Denis Villeneuve and everyone else who worked on the film.

鈥淭here are so many people behind this prize. It鈥檚 a collective prize and I was so proud for them and for us.鈥�

鈥淎rrival鈥� stars Amy Adams as a linguistics expert who tries to communicate with aliens that have landed in 12 pods on Earth.

Jeremy Renner plays theoretical physicist Ian in the Quebec-shot film, which also stars Forest Whitaker as a military leader.