Scenario: I'm browsing my forums via the New Posts page (which displays all unread posts). I finish viewing all the posts I care to read in my New Posts feed then mark all posts read.
Problem: In the seconds between last loading the New Posts page and marking all forums read, new posts are submitted which are marked as read.
Solution: Much in the same way the quick reply window offers to show me new replies in a thread which were posted while was typing up a reply, the "Mark all forums read" dialog could notify me that new posts are present which have been posted since my last page load with a link to the New Posts page.
There are various other approaches to this like having a refresh on the New Posts page similar to Google Plus feeds or the aforementioned quick reply notification which I'm sure could get quite resource-intensive.
If anyone has any other possibly solutions to this I'd love to hear them as well. If this has already been suggested and you know the thread title please point me in the right direction so I can like/contribute. I was not able to find anything related.