Turn on developer level debugging. Enable 'Subsections' in 'Site administration' -> 'Activity modules' -> 'Manage activities'. Set 'moodlecourse | maxsections' (in 'Site administration' -> 'Courses' -> 'Course default settings'. Create a Custom sections course with 9 sections. Turn editing on. Confirm that you can add both subsections and sections. Add a sub section to a section. You should see that there is the message 'You have reached the maximum number of sections allowed for a course.', however you can still see the '+' to add a new subsection. Refresh and observe that the ability to add a subsection is removed and replaced with a pure 'Add an activity or resource' button. Delete the subsection. Observe that you can't still add a subsection or section even though the number of sections is now nine as the subsection (the tenth) is no longer there. Refresh the page. Confirm that you now have the '+ Add section' button at the bottom but still can't add a new subsection even though with nine sections and ten max you should. Click on '+ Add section' and confirm that the 'maxsectionslimit' error with 'Cannot create new section as it would exceed the maximum number of sections allowed for this course ({$a}).' occurs (note that there is also a string bug here with the data for the string not being passed in). Examine the database and see that in 'course_sections' table the deleted subsection (with 'mod_subsection' for the component') is still there for the course even though it was deleted. Examine the database and see that in the 'subsection' table the deleted subsection is still there even though it was deleted.