Hardware and software faults increasingly surface in today’s computing environment and vast theoretical and practical research efforts are being devoted to overcoming the effects of malfunctionality in the computing process. Most research to date, however, has focused on how to discover and handle faulty data. We initiate a formal study of faulty functionality in a modern multicore shared-memory environment. We introduce a model of functional faults, and study avenues that allow tolerating functional faults while maintaining the correctness of the entire computation. We demonstrate the generality of this model by constructing a robust consensus protocol from functionally-faulty compare-and-swap objects. Additionally, We formally prove (tight) impossibility result for the same constructions.