It's not a bug dude, if I am not mistaken, I think it's one the features of this forum, you can actually edit the subject of the topic while you are in the comment typing area.
I personally do not know how useful this feature is, because I rarely see users make use of it, but I think it's cool to have it just incase...

With it, users are able to start a new subject or discussion, inside an already existed and active subject, but then, the new subject must be related to the parent subject being discussed.
This is my personal conception of this though, I could be wrong.