Ok, had a look around, and noticed that it seems to 'hijack' an article.
I couldn't find the original articles, and then realized after comparing with my running forum that the thread is taken over, and the first 'reply' is the first post in the original article. So, not even the article itself is the 'first' post, as I gather it works so far.
I then noticed that the threads it was hijacking were consecutive, the numbers being:
13579
13580
I've included some images that show what I described before.
As a side point, I also noted I have the option on any forum to add content to the header... something I'm not sure if it's intentional or a bug, just posting here for your info.
Anyways, without getting too sidetracked, I've explained as much as I can.
(I even Rebuilt All Namespaces again to see if that made a difference, but it didn't)