The monitoring theory of language perception proposes that competing representations that are caused by strong expectancy violations can trigger a conflict which elicits reprocessing of the input to check for possible processing errors. This monitoring process is thought to be reflected by the P600 component in the EEG. The present study further investigated this monitoring process by comparing syntactic and spelling violations in an EEG and an fMRI experiment. To assess the effect of conflict strength, misspellings were embedded in sentences that were weakly or strongly predictive of a critical word. In support of the monitoring theory, syntactic and spelling violations elicited similarly distributed P600 effects. Furthermore, the P600 effect was larger to misspellings in the strongly compared to the weakly predictive sentences. The fMRI results showed that both syntactic and spelling violations increased activation in the left inferior frontal gyrus (lIFG), while only the misspellings activated additional areas. Conflict strength did not affect the hemodynamic response to spelling violations. These results extend the idea that the lIFG is involved in implementing cognitive control in the presence of representational conflicts in general to the processing of errors in language perception.