I've started noticing more "hidden posts" in a number of discussion threads beyond as3's journal.

A characteristic in common for many of the hidden posts is that the post (or posts) occur right after a new page is needed in the thread for the post.

So while the post might appear listed as the latest post for a thread in the "Today's Posts" results, and subscribers to the thread get emails about it, the new page with the post(s) hasn't been properly sync'ed completely. If you go to the thread the page count is off by one until another post comes in, things get updated and in sync.