The negative-ion accelerator for the MITICA neutral beam injector has been designed and optimized in order to reduce the thermo-mechanical stresses in all components below limits compatible with the required fatigue life. However, deviation from the expected beam performances can be caused by "off-normal" operating conditions of the accelerator. The purpose of the present work is to identify and analyse all the "off-normal" operating conditions, which could possibly become critical in terms of thermo-mechanical stresses or of degradation of the optical performances of the beam.