This one is an official, scientific observation. But it’s so good, it merits a mention here.
The premise is pretty straight-forward. But, we’re going to paraphrase here, so check the Wikipedia definition if you want the science bit.
Intelligent people are clever enough to understand they are fallible and may sometimes get it wrong; they can doubt themselves and their judgement on occasion, but often in a disproportionate or misplaced fashion.
However, those who may lack the necessary knowledge or intelligence related to the position they hold often have a disproportionate amount of self-confidence; they have little or no self-doubt in their decisions or actions and in fact have mistaken confidence in their own abilities.
In summary; Idiots think they are brilliant because they are not clever enough to understand what Good Guys can spot in an instant.
* All credit to David Dunning and Justin Kruger of Cornell University for their observations and definition of the effect.