The medial prefrontal cortex (mPFC) and ACC have been consistently implicated in learning predictions of future outcomes and signaling prediction errors (i.e., unexpected deviations from such predictions). A computational model of ACC/mPFC posits that these prediction errors should be modulated by outcomes occurring at unexpected times, even if the outcomes themselves are predicted. However, unexpectedness per se is not the only variable that modulates ACC/mPFC activity, as studies reported its sensitivity to the salience of outcomes. In this study, mediofrontal negativity, a component of the event-related brain potential generated in ACC/mPFC and coding for prediction errors, was measured in 48 participants performing a Pavlovian aversive conditioning task, during which aversive (thus salient) and neutral outcomes were unexpectedly shifted (i.e., anticipated or delayed) in time. Mediofrontal ERP signals of prediction error were observed for outcomes occurring at unexpected times but were specific for salient (shock-associated), as compared with neutral, outcomes. These findings have important implications for the theoretical accounts of ACC/mPFC and suggest a critical role of timing and salience information in prediction error signaling.