Beatty and Dunaway mistakenly trusted that the people handing them the envelopes were giving them the right one.
Since Best Leading Actress was already awarded, I have to wonder why they still had an unsealed version of that card not only still lying around, but somewhere where it could be mistakenly handed out to be read. You would think that one thing they would make absolutely sure to get right would be that the presenters had the right envelope.
I wonder if now they're going through all the winners to make sure the right ones were announced and that no other mixups happened?
You can bet this mistake will never be made again! And of course it had to be the biggest award of the night...