Stephen Fry explains the Dunning Kruger Effect

The Dunning Kruger effect is a cognitive bias where low-ability individuals overestimate their abilities. It has been known (formally) since 1999, although I am sure this effect was observed well before then, possibly under the heading 'to stupid to know they're stupid'.
For anyone unfamiliar with this effect, there is a short video (narrated by Stephen Fry) which explains it in the context of current American politics.

But what has this to do with chess? In this case not a lot, but it does relate to an observation I've made over the years. The biggest mistakes we make in chess don't happen when we don't have an answer to the problem in front of us, but when we (incorrectly) think we have the best answer to the problem in front of us. Often a game is lost because the move we thought that worked had a fatal flaw in it, and we would have been better off choosing a less flashy move. Usually this is described as over-confidence, which of course is a manifestation of Dunning-Kruger.
By the way, there is a flip side to the Dunning-Kruger effect. Often people who excel at a task don't realise that what they are doing is difficult for the average practitioner, and assume because it is easy for them, it must be easy for everyone. Anyone who has ever coached chess is probably aware of this (although maybe not consciously!)