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.
What happens to geocoding near the poles
Most location software is built, reasonably enough, around assumptions that hold true for the vast majority of inhabited, populated latitudes. Near the poles, several of those assumptions quietly stop making sense, and any system that has not been deliberately tested there tends to behave in ways that range from mildly odd to genuinely broken.
The clearest example is longitude itself. Everywhere else on Earth, a given change in longitude corresponds to a real, calculable distance that shrinks only gradually as you move away from the equator. Near the poles, that distance shrinks toward essentially nothing, since every line of longitude converges to the same single point at exactly ninety degrees north or south. A coordinate near the pole where longitude has changed by a large amount might represent a physical location only a short walk from another coordinate with a completely different longitude value, which breaks any logic that assumes longitude differences map consistently to distance differences, an assumption that is entirely safe almost everywhere else on the map.
Time zones near the poles present their own oddity. Time zones are fundamentally organized around longitude bands, which makes sense for populated latitudes where people actually live and where the sun's daily arc gives local time real practical meaning. Right at the poles, the sun does not rise and set in the usual daily pattern at all, and the concept of "local time based on longitude" becomes close to arbitrary, since there is no meaningful solar day driving it the way there is everywhere else. In practice, polar research stations and similar outposts typically just adopt the time zone of their supply base or home country for practical coordination purposes, rather than deriving one from their actual longitude, precisely because a longitude-derived zone would be more confusing than useful there.
None of this is a defect in geocoding or time zone systems so much as a genuine edge case where the underlying geographic model itself behaves differently. If you are building something that needs to handle coordinates anywhere in the world without excluding the poles, and this is a genuinely narrow slice of applications, worth testing your distance calculations, your time zone logic, and any longitude-based assumptions specifically against high latitude coordinates rather than assuming behavior that holds true for populated latitudes carries over cleanly. For the overwhelming majority of applications, dealing with populated areas and the ordinary range of inhabited latitudes, this is purely academic and not something you need to design around, but it is worth knowing it exists if your data set could ever include a coordinate that far from the equator.