What sometimes happens on my board is that people logged in using
http://domain.ext and
http://www.domain.ext, and then try to logout using the logout at
http://www.domain.ext.
This doesn't remove the cookie set at
http://domain.ext, resulting in
http://www.domain.ext using that other cookie, and the member still being logged in.
The obvious solution is to redirect
http://domain.ext to
http://www.domain.ext