Sometimes, when I read a thread, or use the "Mark all forums read" option, all new posts remain unread. This happened so far only once on PC, but it happens a few times per week on PS3.
For example, I always click on all threads I want to read and then use "Mark all forums read". I read what I want to, reply, etc and then leave the site, or turn the browser off. When I return, all of the previous threads are marked as unread. This happens even though nobody posted in these threads, or the last post is mine.
Just to make sure, I have cookies and java enabled.