When you don't have practiced (non)answers to use when asked a question designed to trip you up and try to answer honestly ("do you support the Bush Doctrine?") you can find yourself momentarily without words. If you are willing to lie, an answer can easily roll off the tongue.
Actually Gov. Palin did a gotcha with Gibson as when he asked about the Bush Doctrine, he didn’t know there were several.
He needed to put his question into context.