Scenario:

1. Teacher develops a course, applies the Collapsed topics format.
2. Teacher makes a course backup file with course files excluded.
3. Teacher restores the backup into an existing course folowing the Delete all contents , then Restore path.

Towards the end of the process (I got 99% in one case, 94% in another), instead of properly finalising, the restore process throws an error:

Exception - Call to undefined method format_topics::restore_numsections()

Debug info: 
Error code: generalexceptionmessage 
Stack trace: 
•    line 150 of /course/format/topcoll/backup/moodle2/restore_format_topcoll_plugin.class.php: Error thrown
•    line 115 of /backup/moodle2/restore_plugin.class.php: call to restore_format_topcoll_plugin->after_restore_course()
•    line 445 of /backup/util/plan/restore_structure_step.class.php: call to restore_plugin->launch_after_restore_methods()
•    line 106 of /backup/util/plan/restore_task.class.php: call to restore_structure_step->launch_after_restore_methods()
•    line 205 of /backup/util/plan/restore_plan.class.php: call to restore_task->execute_after_restore()
•    line 137 of /backup/moodle2/restore_final_task.class.php: call to restore_plan->execute_after_restore()
•    line 820 of /backup/moodle2/restore_stepslib.php: call to restore_final_task->launch_execute_after_restore()
•    line 34 of /backup/util/plan/restore_execution_step.class.php: call to restore_execute_after_restore->define_execution()
•    line 181 of /backup/util/plan/base_task.class.php: call to restore_execution_step->execute()
•    line 191 of /backup/util/plan/base_plan.class.php: call to base_task->execute()
•    line 168 of /backup/util/plan/restore_plan.class.php: call to base_plan->execute()
•    line 408 of /backup/controller/restore_controller.class.php: call to restore_plan->execute()
•    line 219 of /backup/util/ui/restore_ui.class.php: call to restore_controller->execute_plan()
•    line 143 of /backup/restore.php: call to restore_ui->execute()

After clicking Continue I get:

A required parameter (contextid) was missing

Debug info: 
Error code: missingparam 
Stack trace: 
•    line 497 of /lib/setuplib.php: moodle_exception thrown
•    line 571 of /lib/moodlelib.php: call to print_error()
•    line 35 of /backup/restore.php: call to required_param()

which runs in circles after each "Continue" click.

It's possible that the restore itself completes, but does not end gracefully.