Who's the real idiot?

I hate it when people in books act like idiots. And I don't mean making a mistake, or acting according to character flaws, etc. I mean when the author writes people acting like I don't think actual adults would really act. I especially hate it when it comes out of the blue, unexpected and extremely irritating. As a reader, what am I supposed to do? I can't talk to them and tell them to grow up. I can't quit reading the book, because that would be self-defeating. I have to just suck it up and keep reading, hoping that either there is some purpose or some motivation that I have missed, or that the idiocy will turn out to be only a minor flaw in the entirety of the story. And I was really liking this book, too. Stupid people. Stupid author. Ugh.