On HN, if a comment is well-written and appears to be made in good faith but is (in my opinion) wrong, I neither upvote nor downvote, I reply.

Downvoting a good-faith comment because you disagree with it does no one any favors. It creates a sense of persecution among those who disagree with you, while not actually providing any counter-argument for the benefit of the undecided. Leave the vote counts alone, make your own convincing argument.