The error-related negativity putatively reflects the activity of performance-monitoring processes influenced by motivational factors, and is overactive in certain anxiety states, suggesting that affective factors affect its generation. We examined the effects of emotionally arousing and neutral task-irrelevant backgrounds on the error-related negativity to determine whether an affective context 'mismatch' alters error-related neural processing. Event-related potentials were acquired while healthy participants performed a modified Eriksen flanker task wherein flanker stimuli were superimposed on neutral, pleasant, and unpleasant pictures. The error-related negativity varied as a function of picture valence, peaking both earlier and larger in the context of pleasant backgrounds than neutral or unpleasant backgrounds. Findings support the hypothesis that affective factors influence the error-related negativity, potentially reflecting an affective mismatch associated with performance monitoring.