Most of the stores where I live allow you to scan other stores rewards cards. You don't get points on your account but you still get whatever sale prices that are reserved for reward card holders. Wonder if in your case the store supported this function but there was a null or something similar in the record for that card type.
Most retailers use the same ranges for their rewards cards, UPC-A barcodes starting with 4. So even if not intentional, if their system is configured to allow an unregistered card to receive discounts, doesn't validate registration at all, or the number collides with a legitimate rewards card, you'll receive the sale pricing. Similarly, if you simply use a common phone number like xxx-867-5309, 800-555-1212, the store's phone number, etc., you'll probably get discounts too.
At the store I worked at 15 years ago, any 4xxxxxxxxxx code would give you the loyalty program. No reason to authenticate, they'll give you the "discount" (hint its not really a discount) even if you just ask.
