Article 50 of the Treaty of the European Union is a peculiar institutional construction, as (quasi-)federations rarely designate an explicit procedure of secession for one of their members, tacitly barring that possibility. In this article our main puzzle is why the EU policymakers adopted Article 50, allowing for a potential dissolution or major disruption of the union, as made evident by Brexit. Zooming into the Convention of the Future of Europe in the early 2000s, when the exit clause was proposed for inclusion in the Constitutional Treaty, we identify key actors and map the debate in order to understand the rationales underlying this policy choice. We examine the various – complementary or contradictory – explanations which have been suggested, i.e., that the exit clause (1) mirrored the preferences of the newly joining countries from Eastern Europe, (2) sought to appease Eurosceptic member states such as the UK, (3) constituted a strategic calculation of Europhile actors who wanted to advance other legislation. To provide a comprehensive account of the reasons and perceptions that shaped the decision to adopt the fateful article, we rely on the analysis of documents, the historical press archive, and interviews with key participants. Nonetheless, we take the episode of Article 50 as a point of entry to the broader problématique of the design of the EU as an experimental polity, and in particular crisis prone-ness but also resilience.