Happened again. For me it seems to have something to do with clicking certain links but not being logged in at the moment. This time I clicked on a link to a post in an email from someone reporting a spam post, but I had quit Safari earlier, so when I clicked the link it took me to the log in page. Same as above after logging in, all posts showed unread, but after logging out and back in it was back to normal.
It has never happened to me until I was made to log in after clicking a link to a post.
I'm sticking with my story. It never happens to me unless I click a link and get the Log In window. If I'm not logged in and click on a link in the Main Forum window, or like today, twice, I found that going to my History and going to a link, but not being logged in does the same thing. It doesn't do it all the time, but quite regularly. It never happens otherwise, and I usually don't leave my browser open all day like John mentioned in the thread starter, I'm a compulsive quitter.
I wonder if it has something to do with logging in and out all day.
John, does this happen on Windows for you?