Compliance systems face their hardest test when tax fraud is engineered to look procedural, repeatable, and defensible. This book examines Cum/Ex as a business systems failure, not merely a financial scandal. It looks at how financial engineering, fragmented oversight, and weak escalation paths can turn legal ambiguity into operational advantage. Compliance systems appear strong on paper, yet fail when accountability is dispersed across desks, advisors, regulators, and executives. The book explores three mechanisms: incentive design that rewards extraction, governance routines that normalize complexity, and enforcement delays that weaken deterrence. For leaders, investors, and policy-minded readers, Cum/Ex becomes a case study in decision quality. It shows how business ethics collapses when profit logic outruns institutional control. In European markets, the strategic implication is clear: trust in finance depends not only on regulation, but on whether organizations can detect moral risk before it becomes fiscal damage.