Yes, to paraphrase Mr. Rumsfeld, when estimating large system changes you can see the known knowns and have an idea of the known unknowns but the unknown knowns

Yes, to paraphrase Mr. Rumsfeld, when estimating large system changes you can see the known knowns and have an idea of the known unknowns but the unknown knowns and unknown unknowns are hiding like the underwater part of an iceberg. Bad estimates are usually caused by not taking into account the unknowns. How is that for a mixed metaphor?