One big, directly related mistake, in my opinion is the strongly held idea that some people are geniuses, and that general intelligence is a thing.

The more I look into it, the more I see that some people are experts, even genius-level experts, sometimes in several fields, but this level of expertise is a weak predictor of their competence in different fields.

A second big problem is that some people are experts at manipulating masses and at building hype, it can be dangerous because there is a strong compounding effect with hype. (Aka snowballing)