Looking at a clone of 8.x dev with Overlay enabled and see unexpected behavior. After clicking on 'Save' the display goes back to a 'non-overlay' display of the 'Extend' page rather than to the overlay display. This does not happen consistantly but frequently. Also, when returning to the Home page from the admin menu I occasionally see a white screen that lasts 1 or 2 seconds and then the home page shows as normal. Again, these behaviors may be hard to reproduce but the 1st is one I occasionally see in drupal 7 as well.
Env: Apache/2.2.22 (Unix) PHP/5.3.15 mod_ssl/2.2.22 OpenSSL/0.9.8 OS X 10.6.8