Another thing that's undocumented is that comments can be "detached" from a post. The comment isn't deleted, but is removed from view, so the participants can continue having a conversation but that thread is no longer discoverable from the post that spawned the discussion.
Is it no longer visible, or just moved to the top level? I've seen plenty of top-level comment threads that start with something missing context, and then is followed by a reply from dang, of the form "We've detached this thread from $url because $reasons".
