unread posts and mark as read

[#1]

Whta have I done to upset the forum s/w? I inadvertently selected the 'mark all messages as read' and now it appears there are never any unread messages. I thought marking unread messages would only apply to those already posted not the new ones. Maybe I'm missing something here? Well not for the first time

[#2]

you are correct in that it will (should?) only mark those as read, messages which are unread at that time. Anything since you did that should now appear in unread posts.
I've never had any issues with it.