The delivery integration passed every test I had. Then real shipments started coming back with rates the carrier wouldn't honor.
My machine was fine. The problem was that my machine had only ever talked to the carrier's sandbox.
Sandbox responses are curated, with every field present and every status clean, and the calls come back fast. I built my fixtures from those responses, wrote tests against the fixtures, and shipped something that was green end to end against a fiction.
Production sent back a different shape of the same object. Fields the docs called optional were just absent. A rate quote would succeed, the label call behind it would fail, and that left a shipment half-created. Calls timed out and then quietly completed on the carrier's side.
It never looked like an outage. It looked like warehouse staff reconciling shipments by hand, and support answering tracking questions the system couldn't answer. That is what environment drift actually costs you: humans permanently absorbing the gap, with nothing that ever registers as downtime.
So here is what I do now, on every multi-carrier and verification provider integration. I capture and store real production responses as the fixture set. The vendor sandbox I treat as documentation rather than an environment. Every new field shape production reveals becomes a permanent test case, and anything unmatched gets flagged instead of guessed.
I wrote more about how I structure that capture-and-replay layer on my blog: https://polycratia.com/c/28e76d00
If you run provider integrations, I'd be curious what exposed the gap for you first: missing fields, partial failures, or timeouts that later succeeded...