Found a neat bug introduced by the recent fix in http://tracker.moodle.org/browse/MDL-4782 which is harmless but scares the hell out of teaching staff.
1. View a course as a teacher.
2. Collapse the view to show just one topic.
3. Turn editing on.
-> All the 'hidden' topics show as 'orphaned activities' topic, which is incredibly confusing for teaching staff as they are not orphaned, they are purposely hidden.