1. Create a course.
2. Under Course administration, click Backup
3. On the Initial settings step, click Cancel.
4. The following debug message is being displayed:

   You should really redirect before you start page output
   line 902 of /lib/outputrenderers.php: call to debugging()
   line 2739 of /lib/weblib.php: call to core_renderer->redirect_message()
   line 279 of /backup/util/ui/base_ui.class.php: call to redirect()
   line 99 of /backup/util/ui/backup_ui_stage.class.php: call to base_ui->cancel_process()
   line 136 of /backup/util/ui/base_ui.class.php: call to backup_ui_stage_initial->process()
   line 113 of /backup/backup.php: call to base_ui->process()

Though this might not occur in an actual production environment, I think it would be better to have this properly dealt with.