The following scenario may never really happen, but since I'm not sure...

A customer reports an unauthorized transaction with a CSR by telephone. The CSR obtains all the necessary information regarding the alleged error.

All the information is forwarded to the ACH Department to investigate. The ACH Department determines that there is an error (not sure how, not involved in the process) but the customer never completes a Written Statement Under Penalty of Perjury or provides notice of the error in writing.

Must the customer receive credit for the unauthorized EFT even though we cannot return the item without the document?

Take a look in the Infovault section of BOL--lots of good information. This may be somewhat applicable (granted, it was the first item I came across that sounded similar; there may be more):
http://www.bankersonline.com/compliance/guru2012/gurus_comp073012c.html

Thanks!

Although the cited Q & A is on the topic of debit cards, my answer would be the same for ACH. Your ability to return an item/recover funds is separate from your obligation under Reg E to investigate and correct an error. If you cannot return an ACH item because you are missing a form, your only other option is to contact the ODFI for a copy of the authorization and request permission to return the item without the form.

This was changed to Written Statement of Unauthorized Debit a couple of years back. Without this completed by the customer, NACHA Rules does not allow you to perform the return. This is one of those gray areas where Reg E and NACHA are at odds...

Thanks everyone for your help!