Final answer:
Amazon's database storing an order with a non-existent customer ID indicates a referential integrity violation, usually due to improperly enforced foreign key constraints or unpropagated deletions.
Step-by-step explanation:
If Amazon's database has stored an order with an associated customer ID that doesn't exist, we are likely dealing with a case of referential integrity violation within a database system. Referential integrity is a property of data which, when satisfied, requires every reference to an entity (such as a customer ID) to be defined and exist. This situation typically arises when the foreign key relationship constraint is not properly enforced, or there has been a deletion of a record that is still referenced elsewhere without the necessary cascading updates or deletes.