You have a form that accepts payment, for example a donation form. You enter all required information, including the billing information. Within the billing information, the security code is entered. You don't click submit yet, and make a change to the form. For example, perhaps you add another donation with a new designation or change the gift from one-time to recurring. When you scroll back down the page, you notice the Security Code is missing or has disappeared.
This is a security enhancement in the product. Please reenter the security code.