There is a weird bug i 've seen around. Lets say User X (in this case me) reads all new posts and the "ED" icon goes in gray, so far so good. But if this user post a comment, he sees the icon in Blue (as it's like the subforum have new posts) which seems to be bug. Here is a picture showing the bug:
As you can see the icon is in blue. I've checked all the posts and it was gray. Then i post my comment and it turns into blue indicating for new posts, but there are no new ones.