I think it still speaks volumes to HN's credit that a lot of wrongful downvotings are self-correcting. On almost any other site, message board, subreddit, or commenting section, you'd expect to see downvotes' becoming a self-fulfilling prophecy every time. The second something is 'in the grey,' it stays there, and gets worse. On HN, certain measures prevent that from happening -- but people also go to the trouble of upvoting things out of the grey, which is highly unusual by Intenet standards.
It's interesting to look at the group psychology of downvoting. The second something appears grey, a threshold has been crossed. A signal is raised. Everyone else who swings by sees a sort of implicit permission to downvote the comment, because it's already been downvoted, and hence, something about it must be worth downvoting. Before someone even reads the comment, they approach it with a downvote-ready mentality. You're conditioned to react negatively to a grey comment even before you read it, and that's a very powerful prompt.
HN self-corrects in a lot of cases, and that's interesting not only in the context of the Internet at large, but also in the context of human psychology. To read a greying comment with an open mind, and then go against the evident grain to re-upvote it, requires conscious and considered thought. It requires the activation of what Kahneman would call "System 2."[1] Most people don't vote in System 2; they vote in System 1.[2] HN seems to be different, and that's no small feat. It gives me hope, even if the trendline appears to be getting worse.
At the same time, I'm wondering if it's time for HN's admins to raise the karma threshold required for downvotes. Perhaps 500 (the commonly assumed number) is too low?
[1] http://en.wikipedia.org/wiki/Dual_process_theory
[2] I'd love to see a psychology study of internet commenting, whereby the study uses eye-tracking to determine if people actually read comments before casting a vote. My hypothesis is that upvotes usually follow the reading, while downvotes often precede the reading. Particularly if something is already grey.