Assumptions: often traced to be the root cause of a disaster (in engineering)

After leading well over 1500 optical projects for over 500 companies during the last 20 years, some fundamentals emerge. In reviewing the most striking and in many cases devastating failures a clear pattern emerges. In every case, the failure can be traced an assumption that may have been valid for decades, or even centuries, but, is suddenly no longer valid, and the change in status is not recognized in-time.