What's the biggest mistake CEO's make?

Becoming CEO :p ;)

Allowing the power and prestige to go to their head. The best CEOs keep their humility.

Not listening when people say things that they don't want to hear.

Having a mustache without curls at the ends

CEO is a very different job depending on the size of the company. Is this a question about startups? Large mature companies? Something else?

Not gutting the company quickly enough to satisfy shareholder short-term financial gain.

Opening their mouth, generally