I'm in a zip code that spans two cities and I can think of other ways it gets complicated in different scenarios. There are places where I can throw a ball across multiple zip codes, sales tax jurisdictions, municipal lines, county lines, etc., with varying degrees of overlap.

The problem I've anecdotally seen with autocomplete is there's another property with the same street address as mine but in a different city and different zip code (granted only last digit is different IIRC) about ~15 miles away and on more than one occasion it's caused a few mix-ups.