It's been there a while but in the past it would be where you tried to use the same subject more than 3 or 4 times.
Now it seems to be immediate. I can understand why. It makes it just that much more difficult for spammers to copy/paste subject lines when they visit the forums.
Is this a new feature,http://img133.imageshack.us/img133/1841/subiectee3.jpg
It says that "OK" is a duplicate subject.I tried to use "OK" as the subject in the forth post in this thread,http://img101.imageshack.us/img101/4873/subject2we7.jpg