From NBC's Mark Murray
Gravel: that he wouldn't hold his opponents' youth and inexperience against them
Kucinich: fired his police chief, while mayor of Cleveland, on the 6 o'clock news
Clinton: health-care reform and believing Bush's claims that Iraq had WMD
Obama: not fighting hard enough against the congressional effort to keep Terri Schiavo's feeding tube in
Biden: overestimating the competence of the Bush administration
Edwards: voting for the Iraq war authorization.
Dodd: voting for the Iraq war authorization
Richardson: his impatience
An update in response to reader comments: Just to clarify, Obama said his biggest mistake was not trying to stop the GOP-led effort to keep Terri Schiavo alive. He added that such decisions should be left up to the families.