The principle that people will rise to the level of their own incompetence. People get promoted by being good, but not when they're lousy at their job. The Peter Principle states that people will be promoted until they enter a job that they are no good at, and there they will stay forever.
That's why every career manager is terrible at his job.
Bob made middle manager thanks to the Peter Principle.
Description of phenomenon by which the most incompetent members of a given organization will inevitably, inexorably, incomprehensibly rise to the the highest levels of power. In short, a person's responsibilities are inversely related to that person's ability to perform said responsibilities.
George W. Bush's Presidency is the Peter Principle in its most horrible realization.