Unknown Unknowns
quote
Unknown unknowns: These are the least obvious risks, which require a concerted effort to uncover. For example, maybe something elsewhere in the organization or in the industry could dramatically change this project (like budget cuts or an acquisition or new product announcement). Even if you identify an unknown unknown (turning it into a known unknown), you still remain unsure of its likelihood or consequences.
From
Chapter:
Decisions, Decisions
Section:
Beware Of Unknown Unknowns