A high-flying accountant caused chaos at the Oscars after handing Warren Beatty an envelope from the wrong pile, a boss at the firm responsible for the ballot has said.
The climax of the Academy Awards fell victim to "human error", leading to La La Land being mistakenly named the winner of the top gong, according to Tim Ryan, US chairman and senior partner at PricewaterhouseCoopers (PwC).
The cream of Hollywood, along with millions of viewers around the world, were left squirming in their seats as the hit musical's producers handed over the statue to the real winners, the people behind Moonlight.
Emma Stone, who was named best leading actress for her turn in La La Land, reportedly described it as "one of the most horrible moments of my life".
Mr Ryan told Variety magazine that moments earlier Brian Cullinan, one of two PwC accountants responsible for the presenters' envelopes, gave Beatty the wrong one as the Hollywood star took to the stage with Faye Dunaway.
"There's a stack for the back-ups and the ones that are not the back-ups and he took from the wrong stack," he said.
Mr Ryan told the magazine Mr Cullinan, chairman of PwC's US board, was left feeling "very, very terrible and horrible" and repeated the firm's apologies for the historic mix-up.
There's a stack for the back-ups and the ones that are not the back-ups and he took from the wrong stack
The global accountancy company, which has held the coveted role of overseeing the ballot for 80 years, swiftly launched a post-mortem of the incident on Sunday night.
It was reportedly the fourth year Mr Cullinan had been responsible for handing out the envelopes, the contents of which are known only by the two PwC employees handing them out on the night.
Mr Cullinan, standing stage left, and colleague Martha Ruiz, in the wings, each had a pile of envelopes for presenters entering from their side of the stage, Mr Ryan told Variety.
The accountants also had a "back-up" of envelopes for their colleague on the opposite side.
It was from this pile that Mr Cullinan mistakenly picked up the best actress envelope and handed it to Beatty.
Mr Cullinan and Ms Ruiz were forced to intervene on stage amid the confusion and chaos.
Stone told the Daily Mail: "(It was) One of the most horrible moments of my life. Wow! It was just so horrible."
Moonlight, which follows the life of a black boy dealing with his sexuality, scooped three prizes - best picture, best adapted screenplay and best supporting actor for Mahershala Ali.