I've seen some accepted answers on Stack Overflow with negative votes, and the same happened to me yesterday: someone just downvoted my answer, and the OP accepted it without upvoting it.
Wouldn't be a good idea to have a badge for that? Perhaps a bronze one, and a one time thing.
Edit: I was taking a look at this answer, where TheTXI proposes the same, and most of the comments are correct. Perhaps the badge should be awarded only if the answer was already accepted with negative votes.