Testing location data with edge cases, not just happy paths
An address in a well mapped city center tells you almost nothing about how your system handles a rural route, a disputed border, or a query near the poles. Test the hard cases deliberately.
Why postal code boundaries and administrative boundaries differ
It is a natural assumption that a postal code should fit neatly within a single city or county, since both feel like they are describing the same underlying geography from slightly different angles. In practice, postal code boundaries and administrative boundaries are drawn by entirely different authorities for entirely different purposes, and there is no structural guarantee that either one nests cleanly inside the other.
A postal code area is drawn by a postal authority around the practical logistics of mail delivery, typically optimized around delivery routes, mail volume, and operational efficiency for the postal service itself. An administrative boundary, a city limit, a county line, a district border, is drawn for governance purposes entirely separate from mail logistics, often shaped by historical settlement patterns, annexation history, or deliberate administrative decisions made independently of, and often long before or after, any postal code system was introduced in that area.
Because these two systems are drawn independently, by different bodies with different priorities and updated on entirely different schedules, a single postal code can genuinely span parts of more than one city or county, and a single city can genuinely be served by more than one postal code, sometimes with a boundary that cuts through it in a way that looks arbitrary from an administrative point of view even though it makes complete sense from a mail routing point of view.
This has direct, practical consequences for anything that tries to infer administrative location purely from a postal code, or vice versa. Assuming a postal code maps to exactly one city, or that a city maps to exactly one postal code, will produce wrong results in every case where the boundaries do not align, and this is common enough in many countries that it should be treated as an expected occurrence rather than an unusual exception worth special-casing separately every time it comes up.
The more reliable approach is to treat postal code and administrative geography as two separate, independently maintained pieces of location data that happen to overlap in most but not all cases, and to query for administrative components directly, city, region, county, rather than assuming they can be reliably derived from a postal code alone. Our postal code lookup returns matching administrative components directly alongside the coordinates for exactly this reason, so you get the actual administrative geography for a given code rather than needing to infer it and risk being wrong in the cases where the two systems do not line up.