I have a client that experiences this once every couple of weeks. He is a full-time blogger and is frequently logged in and active in the admin for an entire work day.
It is limited to him, and I think it goes away with a full browser reset.
If it happens again, I'm going to spend some real time looking into it. But maybe this info will help someone discover the cause.
If you experience the issue, Figure out a way to dump all of your cookies. DO NOT POST THE LOGIN OR ADMIN COOKIES, as that could give someone access to your site. We want the settings one.