Why would just adding a product to cart create a payment with method type "unknown" and credit card of "4111--------1111" We have the Payment Method Example module turned OFF. And, we only have one payment type set up (Authorize.net) in the Enabled Payment Method Rules.

Yet, when we add just one product to the cart and click submit (without going to checkout, so without filling in any payment fields) we can go to orders/shopping cart and see that a payment with the following fields has been added to the order in process:

Date: "6/25/2015" (even though today is 6/22/2016)
Method: "Unknown"
Remote ID "-"
Result Message: -    Number: 4111--------1111  Expiration: 06/2015
Amount: $1,788 (which is $4.00 less than the total for this particular order. Note that this amount will vary, though.)

This causes the following Authorize.net issues:

1) In the above example, Authoize.net comes in and accepts a transition for $4.00. The order balance is then zero, based on the above $1,788 "unknown" payment plus the $4.00 from Authorize.net. 

OR

2) Authorize.net comes back with the below error when the "Unknown" method payment is larger than the actual order:

REJECTED: A valid amount is required.
AVS response: AVS not applicable for this transaction
CVV match: -

If it helps, here are the rules that fired on the product submit page:

Commerce Pricing Attributes (default)
Calculate taxes: Sales tax (default)
Calculate taxes: VAT (default)
Reset the cart order status on product add or remove (default)
Add to Cart message (overridden)
Redirect after product add to Cart (custom) - a page redirect after submit

So - I need some help to debug/fix this:

1) Is there anything obvious I should look at? I have analyzed the setup of all modules, rules, etc.

2) How would what looks like an example payment type be added before even going to any checkout page - when we have the Payment Method Example module turned OFF.

3) And, since there is no error message - where do I even begin to debug this (I am a novice)?

4) Will Devel help? It seems you need some code added to a module. How do I figure out what module, what code, and where to add the code?

Thanks! Any help you can give would be highly appreciated!
Fixed it!

When you delete an order on the order page, the associated payment does NOT get deleted. And, once the order is deleted, there is no way to delete the payment in Drupal commerce, since you only have a view of the orders.

Then, when you create a new order, the order number you deleted can get 'reused'. So, the old payment is then associated with this new order. That is why we saw the errant payments on the new orders.

So, I fixed this issue by going into the database and manually deleting all of the errant example payments in the commerce_payment_transaction table. So far, so good…. 

Bottom line: NEVER delete an order once live. Or, minimally, always delete the order's payment FIRST, then the order.