Abstract: Hardware and software faults increasingly surface in today’s computing environment and vast theoretical and practical research efforts are devoted to ameliorate the effects of malfunctionality in the computing process. Most research to date, however, has focused on how to discover and handle faulty data. In this paper we formalize and study faulty functionality in a modern multicore shared-memory environment. Functional faults have been previously studied in the architecture community. However, they have never been formally defined and lower/upper bounds were not previously proven. We present a model of functional faults, and study avenues that allow tolerating functional faults while maintaining the correctness of the entire computation. We exemplify this model by constructing a robust consensus protocol from functionally-faulty compare-and-swap objects. We then show a (tight) impossibility result for the same construction, when the number of faults exceeds a certain threshold. Interestingly, for some fault types, more functional faults can be tolerated than the analogue data faults, beating an impossibility result for data faults and demonstrating the difference between the two models.