First unread post doesn't work

[NeoDevin]

Sometimes when clicking on button to take me to the first unread post, it instead takes me somewhere farther in the thread. Posts before that are still highlighted with the reddish icon to say that they haven't been read, so it's only a real nuisance if it jumps me to a later page, because then when I switch back a page, they are all marked read.

I'm using Firefox on Ubuntu if that helps.

[berkeman]

I see that sometimes too, but have just put up with it. Not sure what the algorith is...

I'm on Vista with IE7.