IIRC it had something to do with the parent hierarchy of my neighborhood. It was parented by both a postal code and a city, or something. I'm probably making that up, but I'm sure you can imagine how difficult it is to create and maintain abstractions for every possible geographic situation.

I wasn't aware of any other occurrences.

Not only a zip code can span cities, a single city/zip can span multiple tax authorities e.g. parts of Redmond, WA zip code 98052 are under Regional Transit Authority and others are not.

I think that pretty much any assumption we're making about strict hierarchy are bound to be broken at some point.

I worked with zip codes on a project. Zip codes are a nightmare. They cross everything. There are zip codes which cross state-lines. Zip codes which are not a single contiguous area of land. So many issues.

I feel your pain. It turns out there are even a group of ZIP codes that apply only to specific businesses, buildings, or even people (if you're important enough).

That's because zip codes aren't defined by an area, but by postal routes.

Zip codes don’t denote geographic regions; they denote nodes in the postal routing network.