A post to that thread has been made while you were typing ...
I don't know about others, but sometimes when I'm doing a post on a ship, and hit post, I discover that someone has typed a response to that same thread and posted it just before I completed mine. I find that rather annoying :)
I've been in some other forums where we get a warning message if that happens along the lines of 'While you were typing another user has replied to that same thread. You may want to review your post' and then the other completed post appears in its entirety in the appropriate place in the thread, allowing you to modify yours if you so desire to incorporate their post or respond to issues they raise.
Something like that might be a nice thing to have at some time down the track.
Certainly not urgent or anything like that, of course and there is the D6 and other issues that are far more important and pressing.