It seems to only show up if I have posted in a thread and read all the replies. If there are new replies, the correct icon is there.

http://i28.photobucket.com/albums/c246/muhlissauhh/ergh.jpg

Those are my stickies in hot deals but it happens in the Lounge too.

I guess this is a better example, you can see that the one with new posts doesn't show up like that.

http://i28.photobucket.com/albums/c246/muhlissauhh/argh2.jpg

Have you tried cleaning your browser cache yet?

Also, when it happens again, try to do a complete page reload, using Ctrl + F5 (in most browsers) or Ctrl + Reload button. See if it helps.