What key lesson regarding verification systems did the Hubble flaw highlight?

The need for 'fail-safe' verification and redundancy testing

The Hubble incident served as a critical case study emphasizing that the processes surrounding manufacturing, especially verification and calibration, are as vital as the actual component fabrication itself. The underlying flaw was not just the mirror's shape but the failure of the testing system to catch the error, stemming from the incorrectly positioned null corrector. This demonstrated the danger of relying on a single, non-redundant measurement tool. The profound lesson learned was the absolute necessity for 'fail-safe' verification, meaning that multiple, independent testing methodologies—such as cross-checking the null corrector readings against a different system like interferometry—must be employed to ensure system accuracy and prevent an out-of-specification test setup from validating a flawed product.