Start with an  unpatched Moodle version! Create a top course category "Category 1" Create a course "Course 1" within Category 1 and enrol yourself Create a group "Group 1" within Course 1 In the Course Calendar, click on "Manage subscriptions" Once each for Category 1, Course 1, Group 1, import a calendar (for example the moodle release calendar https://www.google.com/calendar/ical/moodle.com_p4c2oe7hsb77ltaro5qtihb5d4%40group.calendar.google.com/public/basic.ics ) Go to Site Adminstration > Courses > Manage courses and categories Delete Category 1 and choose "Move content to another category" Delete Course 1 Run UPDATE mdl_event_subscriptions SET pollinterval = 1; in the DB, so you don't have to wait. Run cron Verify that the output for \core\task\calendar_cron_task shows an error for each of the three calendar subscriptions Verify that the SQL-Query SELECT id, name, categoryid, eventtype FROM mdl_event; still return events for the deleted category. Now patch and upgrade Moodle. Run cron Verify that the output for \core\task\calendar_cron_task does not list the Calendar Subscriptions anymore. Verify that the SQL-Query SELECT id, name, categoryid, eventtype FROM mdl_event; does not return events for the deleted category. Repeat Steps 2 - 11 Verify that the output for \core\task\calendar_cron_task does not list the Calendar Subscriptions. Verify that the SQL-Query SELECT id, name, categoryid, eventtype FROM mdl_event; does not return events for the deleted category.