A tuning‐initiated failure has been observed in silicon avalanche diode oscillators operated in the Trapatt mode. The precise nature of the failure mechanism remains to be established but several possibilities, based on experimental evidence, are discussed. The circuit‐tuning conditions necessary to initiate the failure (burnout) and the physical state of the diode after the failure are described. A gold whisker, which shorts the junction, is found extending from the mounting stud contact. Preliminary evidence indicates that the shape of the junction region, as controlled by the etching process, can be tailored to inhibit such diode failure.