I wish I could provide a more specific example of this, or some event that seemed to trigger it, but here's the best I can do: I have happily used my admin overlay for most of my CMS work for months now in D7 with the Bartik theme. And the other day, mysteriously it became the case that when I am browsing my site, I will "fall out" of admin overlay mode sometimes.
I just am suddenly on a page as an anonymous viewer would be, with neither the overlay, nor the standard edit, track, etc. content tabs of an admin user. I can still hit the back button, to return to it, and I can still directly enter administrative menus, but an occasional page, and not always the same ones, cause me to fall out of admin mode.
Does this sound like any behavior anyone could identify, or help me identify a process in order to identify?