I don't think anyone will ever know how Warren Beatty and Faye Dunaway got the wrong envelope because I doubt anyone will admit to the mistake and risk their job, but if I had to guess, it was someone from PriceWaterhouseCoopers.
Apparently, there are two people with locked briefcases, each containing a complete set of 24 envelopes. Word has it that Beatty was mistakenly given the second copy of the Best Female Actor envelope.
Then, Beatty opens said envelope, probably thinks he's having a stroke because the card doesn't make sense to him so he coldly hands the winning announcement to Dunaway, effectively throwing her under the bus, and confusion ensues.
Jimmy Kimmel, the show's host had the best line though...
"This is very unfortunate what happened," Kimmel said. "Personally I blame Steve Harvey for this."