The authors propose a model to analyze the conditions of emergence of a twin banking and sovereign debt crisis in a monetary union with an institutional framework which is broadly similar to the Eurozone at the onset of the financial crisis. The authors show that when the responsibility of rescuing the banking system is entirely ascribed to domestic governments -–in particular because the central bank is not allowed to intervene as a lender of last resort on sovereign bond markets -–the main tool to fight against a systemic banking crisis (the financial safety net) may aggravate, instead of mitigate, the solvency problems of banks and of the government. Depending on investors’ expectations, the banking system and the government may either survive a negative financial shock or fail together. In this context of negative self-fulfilling expectations, we also analyze the role of credit rating agencies as potential catalysts to the crisis, the authors emphasize possible contagion expects to "healthy" member states through the banking system, and the authors discuss proposed policy options like the creation of "Eurobonds" to avoid the resurgence of such crises.