Why do unread post appear as read posts?

[admranger]

When I'm going through the different forums, I inevitably run into a situation where a post I haven't read (or tens of posts) show up as read vs. unread. What's up with that and is there a way to fix it?

 

[UN4GTBL]

This is a constant problem for me on this site, using different browsers, computers, etc.

NO other site that I am registered does this.

 

[Steevo]

Ditto
I am using Firefox on a Mac