Registration Failure

Hi,
I have a user who is receiving the following page after registration:

Registration Failure
Your registration was failure
Reason
 

Most users are not receiving this and I'm wondering if there is a problem on my customer's end such as a faulty credit card or an unsafe browser. 
I am having a look at the code to see what can lead to the error, but I'm mostly just looking for a solution to this customer's problem and/or a reason why they would get this message with the "Reason" string blank. 

Thanks,
David