Until now, experience with the consequences of longer operation of failed mixed oxide fuel pins in liquid-metal fast breeder reactor has not been available. Mol-7B, an 18-pin bundle, was originally specified and fabricated to test the SNR-300-Mk-Ia concept under extreme operating conditions, especially with a cladding temperature of 973 K. Because of larger uncertainties of flux distribution in the BR-2 reactor in Mol, Belgium, this bundle was irradiated with a rod power that was 20% higher as expected. As a consequence, 17 of the 18 pins failed during the irradiation campaign. The main reason was mechanical interaction of the fuel with the embrittled and corroded cladding and excessive fission gas pressure. Cladding opening was followed by chemical reaction between sodium coolant and the fuel. A partial blockage occurred from the buildup of the reaction product Na3MO4 and from the diameter increase due to uranate swelling. In spite of these severe failures and large fuel release to the coolant, the bundle could be operated with a high nominal rod power without cladding melting and more severe consequences on bundle integrity and on reactor safety.