What I occasionally experience is, if I click a link that someone posts to another post/thread within SSO, I won't be logged into the page that loads or any pages I navigate to from it, but I'll still be logged in if I navigate away from the page that I came from. It also happens if I click an SSO link from Google, I'll be logged out on the page that loads even if I'm logged in normally otherwise.
I noticed at one point a while back that sevenstring.org and www.sevenstring.org seem to keep a separate login. If I sign in without the www and accidentally land on www, it'll look like I signed out, but really I'm still signed in.