Idempotency

An invoice number with an apostrophe broke a duplicate check. urllib.parse.quote() percent-encoded the filter, the API decoded it before parsing, the malformed query got a 400, the finder returned None, and the caller pushed the invoice again.

An ERP API can answer a PUT with a 500 after it has saved the invoice. A client that retries gets a 200 and reports success with two invoices on the books. Idempotency keys and verify-by-GET did not help, because they ran outside the retry loop.