Good point.
Anderson Cooper may not have known, but there's no question several someones in the corner offices knew. It wasn't a mistake. Nor was it a mistake that the first of the "undecided Republicans" in the focus group they interviewed post debate stated that the debate led her to lean toward John Edwards.
But, as you point out, it just further reveals what CNN is. It is not a news organization. They are political activists who, along with the candidate they support, believe deception is a legitimate tool to achieve their agenda.