2010. The IMF designed Greece's austerity program using a fiscal multiplier of 0.5.
The actual multiplier was 0.9 to 1.7 — up to 3x higher.
They predicted GDP would shrink 2.6%.
It shrank 7.1%.
Over six years, Greece lost 25% of its economy.
The IMF later admitted the error — in a working paper, years after the damage was done.
One wrong assumption. One unchecked model. One country's decade.
This is why forecasting needs more than smart people. It needs a system that catches wrong assumptions before they become policy.