What does your contract with the customer say? Most (in automotive especially) have a supplier quality manual which goes to all suppliers, and which details what recourse they have in the case of defective product.
Yes, just one part with a minor visual defect - but if one has been found, are there more? And will the OEM or the final customer (the buying public) agree that it is a 'minor' defect? That is what frequently drives things like this.