I've observed this bug before as well (and those posts are manifesting the same way for me). No idea what causes it, perhaps the same elusive bug that causes the weird ghost articles (that you click on and get 404ed).
I had that issue happen to me once or twice I think, along with an issue where I would post and it would never update in the "last post" area or bump the post into the newest topics section. I noticed that it happened to me specifically when the site would go really slow in loading and in posting the post/reply. I tried to explain it to will over twitter more than a month ago, but I don't think they have figured out the problem at this point.