: Why aren't some threads being marked as unread?

There is currently no exact way to determine what threads have been read by a user.

The system records your last visit to the message board in a cookie on your browser. The cookie is automatically updated when you have been away from the message board for 15 minutes. Furthermore, the same cookie is accessed by all Website Toolbox message boards. Therefore, the last visited time actually represents the last time you visited any message board hosted by Website Toolbox, regardless of what account it was.

If a post has been made after the time you last visited, it will be marked as unread. Therefore, the only posts that are marked as unread are the ones that definitely could not have been read because they were posted after the time you last visited.

The system cannot determine exactly what posts were actually read. It only does an estimate based on the last time you visited the message board.