There is substantial evidence that subliminal stimuli, i.e. stimuli that cannot be perceived consciously, may influence visually guided human behaviour. Two important points require further investigation, namely, the neural bases and the functional capability of unconscious stimulus processing. In this study we aimed at studying one specific aspect of the latter question, i.e. whether unconscious vision can feed into an error-monitoring system in much the same way as conscious vision. We therefore tested whether the event-related potential component known as error-related negativity, which represents the electrophysiological correlate of an error-monitoring system, is produced by unconscious errors. We found an error-related negativity not only for errors committed in a discrimination task with visible stimuli but also for those committed with subthreshold stimuli. Moreover, behavioural analysis showed post-error slowing of reaction time for correct responses following unconscious as well as conscious errors. Thus, the present results provide both electrophysiological and behavioural evidence of an error-monitoring system operating even when stimuli cannot access consciousness.