fundamental flaw
definitions
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A basic, essential, or structural error, defect, or weakness in a system, argument, character, or object that undermines its integrity, validity, or effectiveness.ru: Фундаментальный недостаток
examples
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The theory was rejected because it contained a fundamental flaw in its logic.
Теория была отклонена из-за фундаментального недостатка в ее логике.