I would agree with the above. First, take a hard, long look at your process to see where this nail MAY of entered your process steam (Perform a Root Cause Analysis). This is fundamental in your CAPA process. Once you figure out how it happened, or your best determination of how it may of happened, correct the issue. If it was a structural correction (loose nails falling from the ceiling), take photos as evidence of it being fixed, maybe you added a new procedure to ensure nails are not coming from wooden pallets (Pallet inspections for example, etc.), then the policy/ SOP and training could be used as supporting evidence too. It sometimes can be difficult to determine foreign material root causes, but i would suggest walking your process flow and taking a hard look at how it may of happened. Then describe what you did (as mentioned above) to your customer after assuring them no more of their product is contaminated (which you need to do/ assess). if you already have procedures in place to mitigate metal from entering your process, than that would be one of the first places to start looking/ reviewing.