LOS ANGELES • Under a PricewaterhouseCoopers (PwC) procedure, only two accountants know the names of the 24 winners after their names are placed in two sets of sealed envelopes. The two accountants also memorise the winning names.
Tradition has it that the envelopes are taken separately in two briefcases to the Oscar venue.
The accountants - in this case, Mr Brian Cullinan and Ms Martha L. Ruiz - are driven there separately, in case of an accident or traffic delays.
They then stand at opposite ends of the ceremony venue and hand envelopes to the respective presenters as each category is announced.
Last week, Mr Cullinan told the Huffington Post the procedure for dealing with the hand-off of an incorrect envelope, other than signalling to a stage manager, was unclear.
"It's so unlikely," he added.
Cornell University-educated Cullinan has been with PwC for 32 years and Sunday was his fourth year of handling the Oscars envelopes. He walks the red carpet, where reporters often say he resembles Matt Damon.
He told one TV crew that he had no nerves.
"We've done this a few times," he said before Sunday night's show, "and we prepare a lot."
It was Ms Ruiz who handed the Best Actress envelope to Leonardo DiCaprio, who presented the award to Emma Stone.
As they walked off the stage, DiCaprio handed her the envelope and said: "Make sure you keep this."
Next up: presenters Warren Beatty and Faye Dunaway. It was then that Mr Cullinan handed Beatty the spare Best Actress envelope instead of the Best Picture envelope.
What led to that mistake could have to do with the design of the envelope.
The academy used a new envelope this year, featuring red paper with gold outside lettering that specifies the award inside. That may have made the outside of the envelopes more difficult to read than last year's, which featured gold paper and red lettering.
Mr Gary Natoli, the stage manager, was the one who approached Jordan Horowitz, the La La Land producer, to see that the envelope he was holding was the spare announcing Stone's acting win.
"The guys in headsets were going around with urgency looking for the other envelope," Horowitz said. "One of the guys opens it and it says Moonlight, and I took it - onstage and went to the microphone and said what I said."