The planned entry of the U.S. into the Anti-Counterfeiting Trade Agreement (ACTA) poses a unique Constitutional problem. The problem is that the President lacks constitutional authority to bind the U.S. to the agreement without congressional consent; but that lack of authority may not prevent the U.S. from being bound to the agreement under international law. If the administration succeeds in its plan, ACTA may be a binding international treaty (under international law) that is not a treaty (under U.S. Constitutional law).
Sean Flynn, ACTA's Constitutional Problem: The Treaty is Not a Treaty, 26 Am. U. Int'l L. Rev. 903 (2011).