I up voted it to cancel your accidental down vote. I have never understood why HN does not let people change a mistaken vote.

Stackoverflow comes close to the same thing, with big warnings if you change your vote that you can only do it once. Maybe it's to prevent us from saying "I liked your comment" and then realizing tomorrow you said something offensive and going back and downvoting all your comments out of spite.

I guess. Doesn't seem worth it, but it's the best I got.

Amen. Especially in a world with small touchscreens.