If your testing an online shopping application, then postcodes for rural areas of the UK that can’t get next day delivery is always a good one to check. Depending on the delivery company, some either charge extra or only offer standard delivery (no express or next day delivery). This usually covers areas in the northern parts of Scotland or certain islands like the Shetland Islands.
Always worth checking these postcodes to see how the application behaves. Does it charge the correct amount for delivery? Does it say that next day delivery is available when it shouldn’t be?
Postcodes in Australia are interesting as they encompass whole districts and suburbs, not just one street. For example Sydney City Centre is one postcode - 2000.
One other interesting thing is that large organisations such as universities can have their own postcodes. The University of New South Wales has its own unique postcode - 2052.
#FunFact
One company I worked at used the branch office street address as an error code 3010, don’t ask, but after a recent check there is no office there anymore.