Create a course and enrol a user in it Enable guest access to the course Create a forum in that course. Create a new discussion in the forum Login as the user Attempt to subscribe to the discussion using the JS links in the thread index Unsubscribe again Login as another user not enrolled in the course View the forum Confirm that the subscribe/unsubscribe link is no longer present View the discussion Confirm that the subscribe/unsubscribe link is no longer present Open mod/forum/discuss.php and comment out the is_guest() check before forum_get_discussion_subscription_icon View the discussion again Click on the discussion subscribe link Confirm the exception was thrown