Well, we obviously can't tell you for certain what happened on the night, but this piece of information might be useful to quash some of the rumours.
In an article published in Medium on February 10, Brian Cullinan and Martha Ruiz from PricewaterhouseCoopers (who "know the list of Oscar winners before the rest of the world,") explained what it was like counting Oscar votes and what happens backstage.
The most important paragraph to read, is this one:
Bran Cullinan: The producers decide what the order of the awards will be. We each have a full set. I have all 24 envelopes in my briefcase; Martha has all 24 in hers. We stand on opposite sides of the stage, right off-screen, for the entire evening, and we each hand the respective envelope to the presenter. It doesn’t sound very complicated, but you have to make sure you’re giving the presenter the right envelope.
So there you have it, it is theoretically possible that a second identical card from the second briefcase was mistakenly handed to the presenters on the night, resulting in the confusion.
I say again, there's no business like show business.