@xssfox A common fallacy I see is "2 apartments in the same building have the same postcode" (generally you'll have at least 2 different 9-digit postcodes for even a small apartment building in the US)

Also "there is only 1 form of postcode in a country". In the US there are "delivery point codes" that uniquely identify a mailbox, they are 11 digits long but normally only printed in a barcode (e.g. 10116-800303). The first 5 digits of that are what humans normally write (e.g. 10116), and the first 9 digits are what commercial mailers print in the address block (10116-8003).