From looking at the stats, and my own browser problems, I would recommend a temporary message is put on the home page to delete cookies, or better yet, have the site admin change the cookie ID so everyone get invalidated and ultimately served a fresh hot cookie.
I've noticed the PHP session ID's are being timed out ( probably intentionally for security ) but new ID's aren't issued. After leaving my computer for some time I'll click on a forum link or refresh the page only to find I've been logged out. And then the uri session id is different to what's being sent from my cookie. Which renders the forum unusable until I clear cookies and close the browser to finish the PHP session.