Herefordshire and Hertfordshire are two counties in England one letter but about 100 miles (160 km) apart - clearly not a situation designed to get the best out of an institution such as the Post Office.
I lived in Herefordshire for some years, and at least twice got mail delivered that was addressed somewhere in Hertfordshire. As certainly my house name, possibly my street name, and certainly my post town were unique, the addresses were nothing like mine in any important respect, making one wonder at what passed for mental processes in a succession of PO staff to have got it to me rather than the intended recipient. At least one of them was:
Wrong house name,
Wrong post town,
Wrong post code,
... and of course ...