One should expect that the expected can be prevented, but the unexpected should have been expected. - Augustine Law XLV, Norm Augustine
The notion of Unknown Unknowns is simply an excuse for not looking further into the pool of risk and for not designing a "system" that is Fault Tolerant, when the pool of risk contains a risk that was missed. Any other approach is a failure to properly manage the risks. This approach comes from manned space flight, nuclear power and other domains where people die, either by accident or on purpose.
Failure to learn how to identify and handle risk is a common cause for most IT project failures. The risks to the IT project include all the common mistakes of poor risk management starting with communication failures and all the softer side of management. Moving to the technical risks, and on to requirements, development, training and deployment.
In an IT project there is no such thing as a Unknown Unknown. IT projects are not inventing new physics, flying to distant planets, or pioneering new drugs. They're simply capturing requirements, writing code, testing this code and deploying the system. No one dies, no one is "lost in space." There is no excuse for not having a credible risk management process in IT.
