When the payment system in game asks for an email, if you put in an address that is already in use, but is not attached to your logged in account, you get this error message on the screen and then on trying to navigate to a different page you get the Big Ol' asp error page with stuff like:

Server Error in '/' Application.

Duplicate entry 'xxxxxxx@gmail.com' for key 'Email'

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: MySql.Data.MySqlClient.MySqlException: Duplicate entry 'xxxxxxx@gmail.com' for key 'Email'
Source Error:
[No relevant source lines]

etc.