In randomized experiments, the assumption of potential outcomes is usually accompanied by the joint exogeneity assumption. Although joint exogeneity has faced criticism as a counterfactual assumption since its proposal, no evidence has yet demonstrated its violation in randomized experiments. In this paper, we reveal such a violation in a quantum experiment, thereby falsifying this assumption, at least in regimes where classical physics cannot provide a complete description. We further discuss its implications for potential outcome modelling, from both practial and philosophical perspectives.