Read the speeches Howard Dean gave before the Iraq war, and compare them with Colin Powell's pro-war presentation to the U.N. Knowing what we know now, it's clear that one man was judicious and realistic, while the other was spinning crazy conspiracy theories. But somehow their labels got switched in the way they were presented to the public by the news media.
Why does this happen? A large part of the answer is that the news business places great weight on "up close and personal" interviews with important people, largely because they're hard to get but also because they play well with the public. But such interviews are rarely revealing. The fact is that most people - myself included - are pretty bad at using personal impressions to judge character. Psychologists find, for example, that most people do little better than chance in distinguishing liars from truth-tellers.
More broadly, the big problem with political reporting based on character portraits is that there are no rules, no way for a reporter to be proved wrong. If a reporter tells you about the steely resolve of a politician who turns out to be ineffectual and unwilling to make hard choices, you've been misled, but not in a way that requires a formal correction.