Just found out that the "unread tracking" system works a bit differently with this forum software, and thought it would be a good idea to post it here (it was confusing me for the first moment).

SMF uses, by design, a different system to track unread topics and posts. The system works well and, in most cases, better than in many other forum software systems where all posts are marked as read when your session cookie expires - which is one of the more annyoing things in vBulletin for example - reading huge forums can be a pain, when the admin has set your session cookie expiration to a short value (maybe only 10 or 15 minutes). If you spend more than this time reading a single thread w/o clicking anywhere, the cookie will expire and if you go back to the index, you'll find out that all osts have been mark as read.

One aspect of this can, however, be a little confusing:

For example: You visit the board index and see that the forum "Foo" has new posts or threads in it (the icon left of the forum name indicates new posts). Now you click on forum "Foo" and see that there are 3 threads with new posts. You read one of the threads, then go back to the index page and realize that the "new post indicator" is gone for that forum, but there are still two more unread threads remaining (you will see this, when you visit the forum "Foo" the next time).

First, I thought it must be a bug, but after searching the SMF support forums I found that it isn't and that the "new post indicator" on the index page is meant to work this way: it tells you, if there are new posts since your last visit to that particular forum. So, if you visit the forum "Foo", the software clears the "new post marker" for that forum, no matter whether you really read all of the threads in it.

The better way for finding unread threads is the "Show unread posts since last visit" link on the sidebar which will quickly list all threads containing new posts.

To summarize: In SMF, the terms "new" and "unread" are not the same. "New" posts are actually posts which have been made since your last visit to that particular forum. "Unread" means that you did not yet visit the thread containing the unread posts. Unread posts don't need to be new, they stay unread until you actually visit the thread or mark them as read otherwise, no matter how often you log back into the forum.