On September 4, 2022, Chilean voters massively turned down a constitutional proposal that responded to widely shared criticisms of the 1980 constitution and emerged from a consensual and participatory process. This result is paradoxical because ex ante, the odds seemed largely in favor of changing the status quo. We argue that three factors, which derived from the interaction between rules and political contingency, explain the outcome: a Convention under the control of party-less independents, the exceptional underrepresentation of the political right, and a highly decentralized and public writing process. We extract some lessons from the failed experience that can be useful for countries seeking to deepen democratization through constitutional change and for a future constitution-making process in Chile.