I guess nothing is perfect and that goes double for software. I have noticed that if I "refresh" a page it makes it "appear" that I was logged out. If I click on login, fill out the fields, I'm told I'm already logged in. I've found that if I click on "Board Index" before trying to log back in all is OK.
I noticed in the forum error log this has happened to a couple others so I wanted to give a heads-up.