Robin2:
...who is supremely good at something - Eric Clapton playing guitar...
In Mr. Clapton's case playing guitar is just one thing he is supremely good at.
Henry_Best:
The Dunning-Kruger effect explains that.
In a previous life I had the displeasure of working with a paid "professional" programmer who was convinced beyond any doubt that once his source code compiled it was bug free. We were significantly more productive after he left the team. Instead of having to do our work and clean up his refuse we only had to do our work. He was a case study in Dunning-Kruger. (He was also extremely sexist. Wowsers that dude was annoying.)