I have had it happen twice with Registered Large Letters. Which I think can be the problem - Postie forgets to Card, or customer forgets (or can't be bothered) to collect.

In my cases, two totally different customers.  One was terribly apologetic, had been admitted to hospital unexpectedly and was unable to pick up from PO before it was returned - she was absolutely lovely & offered to pay postage costs again - because she was so nice, I didn't accept her offer.  The other, total opposite, offered no excuse and pretty much demanded I resend at my expense !  I did, just to avoid a neg. but she also made it to my 'Special' list