In 2011, the Appeals Chamber of the UN Special Tribunal for Lebanon purported to identify a customary international crime of transnational terrorism and applied it in interpreting domestic terrorism offences under Lebanese law. This article argues that the Tribunal's decision was incorrect because all the sources of custom relied upon by the Appeals Chamber – national legislation, judicial decisions, regional and international treaties, and UN resolutions – were misinterpreted, exaggerated, or erroneously applied. The Tribunal's laissez-faire attitude towards custom formation jeopardizes the freedom from retrospective criminal punishment, subjugating the human rights of potential defendants to the Tribunal's own moralizing conception of what the law ought to be. The decision is not good for international law or public confidence in its institutions and processes.