I think I'd be on the phone to Capitol One in a New York minute to argue my case.
Firstly, is it now paid? I would call and explain what a great customer you've been, how your credit is unmarred except for this boo boo, and what could they please please please do to help you rectify this.
Then head over to MyFico.com and see what they suggest over there for a situation like this.
What a bummer... I worry about making just such a mistake, too.