Normally when you open a thread, the "new post" color changes from yellow to gray. However, certain threads like 101 fanfic nonos, the PotC3, and morphing word will not mark as read. No matter how many times I open it.

Interesting revelation. My own thread that I started, this one, will not mark as read. It maintains that there is a new post.

I'd try logging out (as in hitting the 'log out' link up at the top of the page) and logging back in; it's likely you've got an expired session ID or something along those lines.

I've just logged in for the first time today, and the PotC thread and the Book Tourney aren't marking. Weird.

Edit: This won't mark either.

Not that I actually understand how this works or anything, but a lot of times if you're having problems of this sort, you're best bet is to clear out your cookies. But someone who knows more about this than I (translation: just about anyone) ought to follow up on this advice.

Deleted all cookies. Lets hope this work. I hate cookies, they always need to be deleted. Grrr.

Edit: It stayed marked! Woot!

More annoying is when everything on the site marks as read when you haven't read anything.

That's often caused by a corrupted cookie, too. Deleting all cookies from Sluggy.net will usually fix that.