[twodave]

The census bureau (very) periodically publishes zip code data (which is where some places get their geolocation info). If you work with enough addresses you’ll find some zip+4s that are wildly far away from where they used to be. There are paid services that have better accuracy, but I’m not sure how they acquire their data.

[bombcar]

Some people don't realize just how much you can "customize" deliverability with the post office, especially if you're big (like a school or large business) - you can have something that looks like your physical address, but is actually really a maildrop/PO Box at the nearby post office.

You can do relatively complex forwarding that would only appear to the end users if they can decode the barcode.