We just upgraded from 2.6 stable to 2.7b1 (tests-passed) yesterday, to test performance related improvements, and immediately found a rather fatal issue regarding Topic permissions.
This is a topic that @rizka (one of our admins) prepared in the Staff area, before moving it to a public area:
As you can see, it now returns a 404 not found. This is not the case, as everyone with admin rights can still access the topic, so actually the topic permissions are now corrupt. It is located on a public category.
With high confidence this error came with the upgrade to 2.7b1 on the tests-passed branch, as regular member posted to the topic couple of days ago, just before the update.
This is a massive issue as we have several topics that the members of the Staff have prepared before moving them to public areas.