1. Go to the Moodle Features Demo Course, and in any assignment.
2. Choose "assignment" on the breadcrumb - you are now looking on the course in one-topic mode.
3. Choose "Moodle Features Demo" on the breadcrumb - you are still in one-topic mode.
4. Logout and login as the same user again - you are stuck in one-topic mode (unless you press "show all topics" icon).
This is more serious bug then it may be seen because I frequently notice that many moodle users even after year of experience don't know about one/all-topic icons. These white squares don't draw much attention. It is often needed to show confused students how toggle all-topics more, and now that will be a lot more often.
I guess this should be fixed.
Now that moodle.org has switched to moodle 2.0 RC, moodlers can experiment for themselves the problem mentioned in this thread and well described by the OP.
Please vote for MDL-23283 if you agree that the Show only topic n/Show all topics feature has become obsolete, is distracting and should be removed from moodle 2.0 altogether.