The operator is one of the main sources of vulnerability in command and control systems; for example, 79% of fatal accidents in aviation are attributed to “human error.” Following Avizienis et al.’s classification system for faults human error at operation time can be characterized as the operator’s failure to deliver services while interacting with the command and control system. However, little previous work attempts to separate out the many different origins of faults that set the operator in an error mode. This paper proposes an extension to the Avizienis et al. taxonomy in order to more fully account for the human operator, making explicit the faults, error states, and failures that cause operators to deviate from correct service delivery. Our new taxonomy improves understanding and identification of faults, and provides systematic insight into ways that human service failures could be avoided or repaired. We present multiple concrete examples, from aviation and other domains, of faults affecting operators and fault-tolerant mechanisms, covering the critical aspects of the operator-side of the Human-Computer Interaction Loop.