Well-functioning error monitoring of the inner and outer environments is essential for adaptively altering behavior, while malfunction characterizes conditions such as obsessive–compulsive disorder (OCD). The underlying brain processing is manifested as Error-Related Negativity (ERN) signal elicited following error comission, and Perceived Error Related Theta Activity (PERTA) signal elicited following detection of discrepancy in the environment. Yet, while enhanced ERN was repeatedly demonstrated in OCD patients and was found to be potentiated among their unaffected first degree relatives, no comparable observations were reported with regard to PERTA. We recorded EEG activity while OCD patients, OCD patients’ siblings (Family), and healthy controls (HC) performed computerized tasks. For the examination of ERN we used the Stroop task and for the examination of PERTA we presented correct and incorrect mathematical equations. Increased ERN (0–120 ms post response) was observed in both the OCD and Family groups, but only the OCD patients’ signal significantly differed from that of HC's. Similarly, modified PERTA activity was observed in both the OCD and Family groups in the N1 peak (65–125 ms post perceived error), but only for the OCD group this activity significantly differed from that of HC. Both ERN and PERTA's N1 are fast occurring peaks, which suggests that OCD is associate with a constantly over-activated detection system that monitors the inner and outer environment and reacts promptly following detection of a mistake. Furthermore, the modified but non-significantly different activity of the Family group suggests that the pathological condition evolves in vulnerable individuals with neuronal predisposition.