Unknown - Unknown Risks

My biggest U-U risk was a province-wide power outage during development. You can plan and mitigate localized outages, it is more difficult to find alternatives in broad-based disasters.

Hi Rami,
agree, you can never identify all risks. The U-U risks are then what is left over, I heard the them residual risks for them.
As risk identification depends on the experience and knowledge you have available about the project, you will have higher U-U risks with less of own capability. Risk checklists or risk breakdown structures can mitigate that risk.
Example: when I did my first project with subcontractors, I just did not understand the implications and pitfalls of contract negotiation, understanding, control etc.. Only the problems that arose as a surprise (that were U-U risks for me), gave me the opportunity to learn.

One example of unknown unknowns, or unexpected or unforeseeable program conditions, was the enterprise-wide firing and consolidation of multiple Chief Information Officer (CIO) positions at my organization. Several CIOs were key stakeholders and enthusiastic supporters of the program I was managing. Our program leadership, myself included, had to navigate the new landscape of "executive stakeholders" to build support for our efforts. We had to take additional steps to communicate the benefits of the program and to gain trust of the new CIOs. In the end, the impact on the project team was minimal.