Registrations are sometimes not created when orders are completed

You can use the following query (or build a similar view) on a Drupal site to identify orders that failed to create their proper registrations.

SELECT co.order_id, co.order_number, co.status FROM commerce_order co LEFT JOIN field_data_field_ticket_order fto ON fto.field_ticket_order_target_id = co.order_id WHERE fto.entity_id IS NULL AND co.status NOT IN ('cart', 'checkout_review', 'canceled');

In our testing the 'rebuild registrations' button does not succeed in restoring these registrations.