A huge one that obliterates all subsequent suppositions in economics: the systemic depravity of reason. For a number is, at best, a consequence, not the cause of economic performance nor carries any meaningful comparative value.
You see, measuring human performance by GDP is like measuring the quality of a soccer game by counting the goals scored. A fallacy in its own right, but it gets worse; it subsequently suggests when another game yields the same score, gameplay must have been identical. Such is the depravity of reason stemming from confounding of consequence and cause, so omnipresent in our voodoo religion of economics.
Human performance is not a mathematical equation. Nor can it be modeled by absolutism. Or can it?