To predict the Rolling Contact Fatigue life of components such as rolling element bearings, it is important to understand the failure mechanisms. A rather unusual failure was observed on integrated races into a planetary gear train. The failure of the planet of this gearbox was attributed to the very long propagation, at an unusual depth, of a crack, which probably started at the surface. The present preliminary study aims to gain a better understanding of the factors that can lead to this type of failure. Firstly, a twin-disc machine is used to reproduce the contact conditions identified in the application. A second set of more severe conditions is also tested. A comparison is made between a carburised steel (16NiCrMo13) and a nitrided one (32CrMoV13). Two distinct crack patterns and spalling characteristics between the two materials are observed. Some unusual crack characteristics are observed in the carburised sample for a specific set of contact conditions.