Their experience, I guess. I know that, of the university professors and professional engineers I know, they are some of the smartest and also some of the dumbest people I've ever met. It's like they invent new ways to be wrong, sometimes. I can easily imagine any of them in a jury room talking complete nonsense about who did what and why and how the law shouldn't be that way or mean that and so they have to acquit--anything but actually listen, follow along, and apply the judge's instructions.

A stupid person can make only certain, limited types of errors; the mistakes open to a clever fellow are far broader. But to the one who knows how smart he is compared to everyone else, the possibilities for true idiocy are boundless.