Thinking before speaking, and thinking before acting should be treated with two separate critiques, One is much worse than the other, as you've pointed out, you can go back on your word, you can easily fix something you've misspoken about. BUT you can't go back on your actions as redily.
now i'm not condoning the fact that the Dems say one thing and then say the opposite, as you've pointed out, but you've got to realize that The Republican's hands aren't clean either. What did McCain say about Iraq? That it'd be super easy, and they'd treat us like liborators, we'd be heros. Well then he said he never said that. But i'm not going to go back and forth over this, you know it, and i know it, people aren't 100% consistant
BUT please recognize that making a decision like being the VP when you explicitly said you didn't know what the VP actually does literally months earlier, requires a thought process, and if you forgo that process, or if you embrace that way of 'not' thinking, then you're doomed.