It can happen to any of us. We're there, minding our own business, probably giving someone advice, and we're struck with the absolutely terrifying realization of our own competence.
It hits us that we know what we're doing. We are struck by the realization of how much we know and what skills we have. Our ability to effectively leverage our history and experience looms large in our mind. We realize we're really quite competent.
This of course, is a terrible blow.