This started fairly reccently, probally within this hour or the previous one.

I have tried clicking mark fourms read and refreshing my cache.

Nothing in the fourm is marked unread, though.

EDIT:
I'm using firefox 3.

It could be that someone has made a post as soon as you mark all forums read.

Ctrl+f5 will refresh while cleaing that pages cache.

Still lit up.

Same.

Been noticing this a few other times recently.

Hitting mark everything read also doesn't do anything.

Same, but mark everything fixes it. It also goes away after maybe 10 minutes.

EDIT: Oh, and just a few minutes ago, everything in the top level Mod Making forum was marked as unread. All the way down the list. Mark everything read fixed it also.

This button should mark everything read..

Well, that button fixes it.

That's because that's the button you should be pressing.

But you see, the forum icon is supposed to tell you when everything has been read in that forum. This is after I've read everything in the forum, and it's still lit up, even after refresh.

Did you also read all of the stuff in the subforums?

Sort of hard now that every thread in the first 4 and a half pages is marked unread unless you've recently read it.

Yes. This happened in Support once too. Every topic was read (the topic icons were dark), subforums and otherwise, but the forum icon is still lit. It's not a big deal, since it fixes itself after ten minutes or so, just a minor annoyance.

Err... if you haven't pressed mark all as read yet try checking the next few pages of Mod Making. Mine had every thread marked as unread for the first 4 and a half pages.

Logging in and out (read: ending your session) fixes this problem.

As does marking all as read.