Answer:
The customer's liability is all transactions occurring after 60 days following the first periodic statement showing the first error. Refer to Section 205.11(b)(1)(i).
Let's assume that the 10/08 statement showed the first unauthorized error and it occurred every month since then. Let's also assume it is $100 per month. You owe them for the $100 in 10/08, 11/08 and 12/08. They are liable for all transactions since then.
You can charge back through NACHA for the last 60 days. This may offset the money you give to the customer, but it often leaves you one month short ($100 in my example).
First published on BankersOnline.com 5/04/09