If you look at the close-ups, it's true that the actor was given the wrong card when he stepped on stage. It indeed read "Actress in a Leading Role"

A stagehand actually realized Beatty had the wrong card after he had stepped on stage. Oh ... Oh my god, he got the wrong envelope," someone from backstage reportedly said, after the presenters had already left the wings for the mic, according to tmz. According to a story published by Slate in 2014, there are two separate sets of the winning cards. This is how they are created: "The head accountants then stuff two complete sets of presenter cards into envelopes, and place them into identical briefcases. (The stationers prepare presenter cards for every nominee, just as both teams in a World Series have championship hats and t-shirts made in advance of the game. The loser's cards are destroyed.) The briefcases remain in a vault until the day of the show. The two accountants each carry one briefcase to the theater, traveling in separate cars. The last thing presenters do before striding into the spotlight is receive the envelope from a PwC accountant, who is standing just offstage.

Stone told Entertainment Weekly: "I also was holding my Best Actress in a Leading Role card the entire time. So whatever story... I don't mean to start stuff, but whatever story that was, I had that card. So I'm not sure that happened.

In addition to announcing the wrong winner in the theater, why did The Academy also tweet out that La La Land won Best Picture? Blank graphic templates are made so that someone can input the winner as soon as it's announced, before tweeting it out, a historian from the Academy explained to BAZAAR.com's Culture Editor, Julie Kosin.