Mark Ruffalo is known for speaking out of turn when it comes to interviews. Not that he says anything to offend people or anything like that, just that he has a way with words that often leads to spoilers being given out.
Recently, when speaking at the Santa Barbara International Film Festival, Ruffalo was asked whether he's coming back in Captain America: Brave New World, Ruffalo said "yeah," but then went onto say he couldn't talk much about it, apart from saying it's exciting.
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Apparently, Ruffalo misspoke, and the story now is that he won't appear in Captain America 4. Considering this is Ruffalo, though, and that Red Hulk very well could be in the movie, a lot of fans are going into conspiracy mode, trying to think of ways Ruffalo could make an appearance.