Thanks for the response DrByte - and your patience; I'm not the sharpest knife in the drawer.

* They paid with credit card.

* There are three modules: 1, the payment gateway (Authorize.net); under Modules it has a green circle and the Orders Status is Pending. 2, Check/Money order; the Order Status for that is "default"
And 3. Ceon Manual Card v.2.2.2 has a yellow circle and is "Processing" under Orders Status

* The customer used a MasterCard for their purchase.

* They got an email from us ("Thank You For You order!" with order information, billing and shipping information, and transaction ID, etc.)

* The identifying #s they were assigned in their email were: Customer ID and Transaction ID. This customer ID and the customer appears under Customers --> Customers in Zencart. However, the customer's name and order doesn't appear on Customers --> Orders.

* In the admin orders screen there is no missing order that would correspond. For example, there were no order totals that matched the "missing" order.

* I don't believe we have any add ons or plugins; but if we didn't, I'm (sadly) not sure how to check.

* I'm using
Database: MySQL 5.1.56-log and PHP Version: 5.4.20 (Zend: 2.4.0) ## PHP Memory Limit: 90M

Lastly, I asked a customer about their experience and they said: The books remained in my basket on the website where they still remain, yet my credit card was charged and I received a notice.

In other words, the products remained in the customer's basket but the card was charged.

Thanks!