You don't know what you don't know: The fear of Unknown

Back in 2009 a new Batch of Software Engineers were being inducted into their Corporate Careers. The question to this particular young first time lead engineer, who was getting a real job after a long gloomy recession of 2008, was whats your biggest concern stepping into the real world. As he blabbered and stammered to find words, the experienced Training Lead, who was also a seasoned industry technology leader framed his thoughts into these words

You don't know what you don't know.

Almost after half a decade into the industry since that day, during a critical Go-Live decision making meeting, a 70-30 success probability prediction was being discussed and were also nostalgically accompanied by those same dreaded words,  

We don't know what we don't know yet.

To me its such an ironic situation as its the most perfect excuse to accept all unseen risks and also the most compelling reason to go ahead just to find what is it that we don't know that we don't know.