A good well-actually here often adds a lot to the discussion: it brings up points that others may have forgotten, elaborates on concepts that can be very complex, points the anonymous reader toward other resources that they may never have heard of, and can lend validation and social proof to ideas. And the negatives are curtailed by the short, high-population, transactional nature of message boards. When someone corrects you on Hacker News, it doesn't make you feel stupid (I hope): it just means that someone out there, among the hundreds of thousands of readers, had better information than you did. There's no real jockeying for social status on Hacker News, because if you get karma or score that point - who cares? You never see them again.

That's very different from a school environment, where you are pushing your comfort zone in front of a small number of classmates, and a correction can make you feel stupid and negatively impact your confidence. You have to learn with these people for a while; if you feel devalued from the beginning, your learning is going to suffer.