Unknown unknowns are in the very center of Project Management.
Project management is the process of making something new. When you do that, you may be just assembling familiar pieces in familiar ways. That’s the simplest activity on the arc that graduates through levels of familiarity (process management, really) into areas of creativity at increasing levels of risk.
Risk is the possibility that what you attempt won’t work, and failure can sometimes carry a high price. So project work often is launched and conducted tentatively. The sponsoring organization bases its approach on wishing and hoping more than on managing, particularly managing risk.
Timidity puts real project success out of reach, and many organizations assume that’s the way it has to be.
Successful organizations, on the other hand, accept a certain level of failure. They learn from their failures, and apply the learning to a broad pattern of success. Their failure is a purposeful, controlled component of their success. They know they can’t grow without it.