Suppose I have a course with 5 sections, each of which contains a single forum. I then go to Settings and change the number of topics to 3. Now sections 4 and 5 are not available on the course home page.
However, if I click on the 'Forums' link on the Activities block, not only can I see the forums in sections 4 and 5 listed, I can even access them and post messages.
This is undesirable - if I have made the forum inaccessible on the front page, I do not want it to be accessible through the back door.
PS while this bug could probably be quite easily fixed, I think the real problem here is the inability to delete sections - see http://tracker.moodle.org/browse/MDL-10405.