Accountant Brian Cullinan, who media reports said had been tweeting backstage shortly before, gave presenters Warren Beatty and Faye Dunaway the wrong envelope for the movie industry’s top award on Sunday, the accounting firm said in a statement.

In a gaffe that stunned the Dolby Theatre crowd in Hollywood and a television audience worldwide, “Cullinan mistakenly handed the back-up envelope for Actress in a Leading Role instead of the envelope for Best Picture” to Beatty and Dunaway, PwC said.

“Once the error occurred, protocols for correcting it were not followed through quickly enough by Mr. Cullinan or his partner.”

The Wall Street Journal and celebrity website TMZ.com reported that Cullinan had posted a backstage photo of actress Emma Stone on social network Twitter minutes before the mix-up.

The photo, from Cullinan’s Twitter account, was later deleted but was still viewable on Monday on a cached archive of the page. Cullinan could not immediately be reached for comment.

The mistake was not rectified until the “La La Land” cast and producers were on stage giving their acceptance speeches. It was left to the musical’s producer, Jordan Horowitz, to put things right.

“Guys, guys, I’m sorry. No. There’s a mistake,” Horowitz said. “‘Moonlight,’ you guys won best picture. This is not a joke.”

It took three hours for PricewaterhouseCoopers, which has been overseeing Academy Awards balloting for 83 years, initially to confirm that Beatty and Dunaway received the wrong category envelope.

PwC said it took full responsibility and apologized to the casts and crews of “La La Land” and “Moonlight.”

“We sincerely apologize to Warren Beatty, Faye Dunaway, (host) Jimmy Kimmel, (broadcaster) ABC, and the Academy, none of whom was at fault for last night’s errors,” it said in its statement.

The Academy of Motion Picture Arts and Sciences, which organizes the Oscars, also apologized for the mishap and said it was committed to upholding the integrity of the Oscars.

“We have spent last night and today investigating the circumstances, and will determine what actions are appropriate going forward,” it said in a statement.

An embarrassed Beatty carried the envelope to the glitzy Governor’s Ball after the show, with the writing clearly saying “actress in a leading role.” “La La Land” star Stone had been awarded that Oscar moments before.

“Except for the end, it was fun,” Kimmel said on Monday, referring to the Oscar show he hosted.

“You know it’s a strange night when the word ‘envelope’ is trending on Twitter,” he said on his ABC show “Jimmy Kimmel Live.”

‘Not Advanced Math’

Brand management experts said it could take years for PricewaterhouseCoopers to recover.

“This is not advanced math. PwC had to get the right name in the right envelope and get it to the right person,” said Tim Calkins, a marketing professor at Northwestern University, calling the blunder a “bit of a branding tragedy.”

Under a PwC procedure, just two accountants know the names of the 24 winners after their names are placed in two sets of sealed envelopes. The two accountants also memorize the winning names.

Tradition has it that the envelopes are taken separately in two briefcases to the Academy Awards venue. The two accountants – in this case Cullinan and Martha Ruiz – are driven there separately, in case of an accident or traffic delays.

For more on the Oscars, watch Fortune’s video:

The pair then stand off stage at opposite sides and hand envelopes to the respective presenters as each category is announced.

Last week, Cullinan told the Huffington Post the procedure for dealing with the hand-off of an incorrect envelope, other than signaling to a stage manager, was unclear.

“It’s so unlikely,” Cullinan added.

The error was corrected quickly, although precious minutes passed, said Anthony Sabino, a law professor at St John’s University in New York.

“It’s not as if we woke up this morning, or if it had been uncovered after the telecast was over. That would have really have been a black eye,” he added.

Compared to accounting fraud at other companies in the past, “this incident diminished vastly to a vanishing point,” Sabino said.

The “Moonlight” filmmakers were gracious about the error.

Director Barry Jenkins told reporters backstage that he received no immediate explanation for the mix-up, though “it made a very special feeling even more special, but not in the way I expected.”

Jenkins added, “Please write this down: The folks from ‘La La Land’ were so gracious.”