UPDATED: In his fourth year of handling envelopes for the winners of the Academy Awards, PricewaterhouseCoopers partner Brian Cullinan picked a bad time for a miscue — pulling the envelope for what was supposed to be the best picture winner from the wrong pile.

As a result, presenter Warren Beatty carried the wrong envelope Sunday night, meaning that his on-stage partner, Faye Dunaway, announced “La La Land” was the winner of the night’s top prize. “Moonlight” was actually the best picture winner.

Tim Ryan, the accounting firm’s U.S. chairman and senior partner, offered an explanation Monday of the unprecedented miscue, along with his profuse apologies to the Motion Picture Academy and to the filmmakers behind the two movies.

“We clearly made a mistake and once the mistake was made we corrected it and owned up to it,” said Ryan, attributing the foul-up to “human error.”

Ryan said he had spoken to Cullinan about the episode at length. “He feels very, very terrible and horrible. He is very upset about this mistake. And as a firm, given that he is a partner of our firm, it is also my mistake and our mistake and we all feel very bad.”

Cullinan was positioned stage left during the evening. His colleague, Martha Ruiz, stood in the wings, stage right, Ryan said. Each had a pile of envelopes for presenters entering from their side of the stage and then a pile of “backup” envelopes for the presenters entering from the other side of the stage at the Dolby Theatre.

Cullinan pulled the lead actress envelope that was in the “backup” pile and handed it to Beatty. “There’s a stack for the backups and the ones that are not the backups and he took from the wrong stack,” Ryan explained.

Beatty hesitated twice after opening the envelope, apparently confused when he saw both the name of lead actress winner Emma Stone and “La La Land.” Not understanding Beatty’s hesitation, Dunaway said: “You’re impossible. C’mon.” Beatty then showed her the card and Dunaway announced: “La La Land.”

Asked if Beatty and Dunaway could have recognized the error, Ryan said: “No, look, it’s our job, not theirs.”

The PwC chairman said that both Cullinan and Ruiz, who had memorized all of the winners, as per company procedure, realized instantly that an error had been made. “Martha … immediately recognized that was not who won,” Ryan said. “She opened up her backup envelope to confirm her memory.” Cullinan immediately told members of the production team of the error, Ryan said.

Procedure for such a mistake calls for the PwC employees to then take the stage, along with members of the production staff, to correct the error, Ryan said. He did not know why it took more than two minutes for the correction to be made, with “La La Land” producer Jordan Horowitz delivering the news to the world that “Moonlight” was the real best picture winner.

“This is the first time in over 80 years that there has ever been a problem and again we are very disappointed in ourselves,” Ryan said. “I have reached out to the Academy and shared with them that we take full responsibility that Brian had made the mistake and the firm takes responsibility for that.”

Ryan said he was not focused now on whether the flub would hurt the firm’s relationship with the Academy or its prospects of returning for another year of counting the ballots. “I expect us to get something like this right and our focus right now is just on making sure the academy and the rest of the folks know that and the rest will play out,” Ryan said. “We are very proud of being associated with the Academy Awards. It’s good for our brand. It’s good for our people. So while I am concerned I hope we will be judged on how quickly we reacted and owned up to the issue.”

Watch the moment unfold below: