Learning abilities depend on detection and exploitation of errors. In primates, this function involves the anterior cingulate cortex. However, whether anterior cingulate error-related activity indicates occurrence of inappropriate responses or results from other computations is debated. Here we have tested whether reward-related parameters modulate error-related activity of anterior cingulate neurons. Recordings in monkeys performing stimulus-reward associations and preliminary data obtained with a problem-solving task revealed major properties of error-related unit activity: (i) their amplitude varies with the amount of predicted reward or the proximity to reward delivery; (ii) they appear both after execution and performance errors; (iii) they do not indicate which error occurred or which correction to make; and (iv), importantly, the activity of these neurons also increases following an external signal indicating the necessity to shift response. Hence, we conclude that anterior cingulate 'error' activity might represent a negative deviation from a predicted goal, and does not only reflect error detection but signals events interrupting potentially rewarded actions.