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There's The Peter Principle, for when someone who's talented gets promoted above their skill level.
Edited by RustBeard
The Peter Principle says that employees at a place of business will get promoted until they're at a position for which they're unsuited. for more general examples, Know-Nothing Know-It-All and Ignorant of Their Own Ignorance cover someone who thinks they're good at something until extraneous circumstances force them to realize they aren't.
Someone thinks they are good businessman until they are placed at the head position. And aren’t as skilled as they are.