This event-related potential (ERP) study explored the behaviour of N400 and post-N400 frontal positivities (pN400FP) during the processing of emotionally biased and unbiased sentences that randomly led to highly expected or unexpected word outcomes. Unexpected outcomes (as determined by sentence completion written tests) elicited significantly larger N400 and pN400FP responses than did highly expected outcomes. Emotionally neutral outcomes triggered a significant N400 expectancy effect across all scalp locations, including frontal sites, whereas emotionally biased outcomes elicited a significant N400 effect localized to posterior scalp regions. The subsequent pN400FP effect was significant only when emotional expectations were violated and not when emotionally neutral sentences led to unexpected outcomes. This frontal effect, linked to the processing of lexically unexpected but plausible words, showed larger amplitudes for unexpected pleasant surprises than for unexpected setbacks. Our results support the view that the pN400FP response to unexpected verbal outcomes entails more than a generic reaction to a lexical ‘misprediction’. Rather, they favour the hypothesis that the affective content of the sentence being processed influences the effort needed to override a lexical prediction, such that more effort is needed to override a pessimistic prediction than an optimistic one.