Under the principle of unjust enrichment, Member States may refuse to repay indirect taxes to traders on the ground that they have already shifted the burden of the tax to a third party and would be unjustly enriched by receiving the tax from the tax authorities. In a large number of judgments, the ECJ has described the terms and conditions under which Member States may apply the principle of unjust enrichment. In this article, the author explains that those conditions are significantly different depending on whether the tax refund concerns a tax that Member States imposed in violation of EU law or VAT charged by a trader to his customer by mistake.