It's been called the biggest blunder in the Academy Awards 89-year history - La La Land mistakenly being named Best Picture instead of the Barry Jenkins-directed Moonlight.
It caused mass confusion on the stage after Hollywood veterans Warren Beatty and Faye Dunaway were handed the Best Actress award envelope for Emma Stone in La La Land.
Almost immediately after the film's stars and crew went up to accept their award, host Jimmy Kimmel had the awkward job of stepping in to announce there'd been a mistake.
Organisers had sought to explain the gaffe, but viewers had their own conspiracy theories about how it happened.
Was it Leo's revenge? DiCaprio was spotted holding the Best Actress award which he'd just announced and handed to Stone following her win.
Some thought it may have been him getting some Revenant-style revenge for only having won his first Oscar for his role in the Alejandro Inarritu-directed epic in 2016 having been nominated numerous times before.
Did Beatty and Dunaway, who played glamorous crime couple Bonnie and Clyde, try to pull off one last swindle?
Others were quick to pounce on a meme of La La Land co-lead Ryan Gosling whispering something into a tourist's ear as she looks on in apparent horror - a moment which happened earlier in the night.
"I switched the envelopes," one person joked.
Another theory was that Matt Damon had something to do with he - he and Kimmel have a long-running fake feud and the host had roasted him at the start of the night about Damon's starring role in box office flop The Great Wall.
"When I first met Matt, I was the fat one. He's a selfish person, but Matt did something very unselfish. He could have starred in Manchester by the Sea, but he gave that role to Casey Affleck," Kimmel joked.
He said Damon went on to star in a "Chinese ponytail movie instead and that movie went on to lose $80 million. Smooth move dumba**."
But despite all the hype, the real reason was perhaps a bit more innocent.
PwC, which tallies the Oscar results, and the Academy of Motion Picture Arts and Sciences issued a joint statement apologising for the mix-up.
"We sincerely apologise to Moonlight, La La Land, Warren Beatty, Faye Dunaway, and Oscar viewers for the error that was made during the award announcement for Best Picture.
"The presenters had mistakenly been given the wrong category envelope and when discovered was immediately corrected. We are currently investigating how this could have happened, and deeply regret that this occurred.
"We appreciate the grace with which the nominees, the Academy, ABC, and Jimmy Kimmel handled the situation," the statement read.
Only two people know the winners of the Oscars and each carry a suitcase with the set of envelopes as they stand on opposite sides of backstage.
When Beatty and Dunaway took to the stage, they realised they still had the two Best Picture envelopes.
But it was too late to fix the mistake.
And the rest, as they say, is history.