When I think of the concept of "over-preparation" vs. "under-preparation", I think of life-long learning vs. Doing less than the bare minimum.
So far that I've noticed, over-preparation can lead to frustrated interview results of "you're over-qualified" which is another way of saying that you're not likely to submit yourself to being someone else's eternal servant. In time, these individuals are faced with more opportunity if they can get their foot in the door.
Conversely, people who don't prepare much at all are faced with more low-level opportunities and table scraps, but unless they are taller and more handsome/beautiful than everyone else or have rich parents, they never go any further in life.