People have credentials, and they have areas of expertise. You get in trouble when you assume that the area of expertise matches the area of the credential.

Example: CEO. If you're a CEO, you know how to run a company, right? So if you know how to run Pepsi, then you know how to run Apple, right? In practice, it turns out, no you don't.

Example: You're an atomic scientist with a specialty in nuclear weapons. People interview you about the danger of nuclear war, because you're an "expert". But you're not an expert on the military, or on international relations, or on game theory. You have an amateur's understanding of such matters - maybe slightly better than the mouthy guy at the bar, but not much. But they're interviewing you for serious, mainstream media because you're an "expert". (Note: I don't have a concrete instance that I'm pointing at in this paragraph, unlike the previous one. None the less, I think that people get interviewed as "experts" far outside their actual expertise. And some "experts" like to stretch their "expert-ness" to add weight to their pronouncements on areas in which they are amateurs.)