Right: if you can't say something useful and accurate, please don't post. (There's still a LOT to say, so don't let that stop you from posting at all!) If you post in good faith and are inaccurate, then it's good form to either edit your post to acknowledge the error, or delete your post.
Getting downvoted is one way that newbies learn about HN; most of the time, the downvote comes with a discussion.
(One last note: sarcasm is really hard to detect in writing; we miss the tone of voice and all the nonverbal cues. It's polite to mark a sarcastic response with something that shows you're being sarcastic. Don't count on just the content of your post to show sarcasm.)
