There have been a lot of cases of this in this election cycle. One of note was Steele's sister's response to the Michael J. Fox commercial about stem cell research. The response was a lot more devistating than the original ad because it used facts to rebut a fairly fact free ad.
It is too bad that John Kerry isn't running this time, because he has, again, opened himself up to negative advertising by inartfully suggesting that only the least educated are fighting now for us in Iraq. It doesn't help his cause that this was very similar to his famous Congressional testimony about alleged atrocities during the Vietnam War - and is one of the reasons he lost in 2004.