Error Code 51.

The answer I got from Nexon when I got this error was they thought I was being suspicious and that I didnt verify the email or something..but I very well didnt know that I was supposed to do that..so in the end they ended up banning my account from charging. Even though they always got their money.

This is the reply I got from Nexon when I asked why my account had been flagged and if they could remove the flag from that account.

 Spoiler

I understand that you are having trouble purchasing NX. Thank you for reporting this information. I can understand how disappointing this must be for you. I am extremely sorry for any inconvenience this has caused, and hope that you can accept my apology.

Please be advised, we have a verification system due to continued fraudulent charges from players. The error you received was because your transaction failed the verification process of this system. I am unable to verify your account or lift this restriction for you.

This account is not able to use PayPal or the credit card system. Please feel free to use our other payment options such as PayByCash or Nexon Game Cards. More information about our payment options can be found here:
http://www.nexon.net/nx/guide/

Thank you for your understanding.

Sincerely,

GM Bartleby
Nexon America Inc.

Edit : This was back in December 2011 when this happend to me.