According to the monitoring theory of language perception, language errors can create strong conflicts between expected and observed representations. When a strong conflict is present this functions as a bottom-up signal to bias attention towards the unexpected representation for reprocessing to check for possible processing errors. This monitoring process, encompassing both the conflict and reprocessing, is thought to be reflected in the late positivity or P600 effect. The present ERP study compared sentences with syntactic agreement violations to sentences containing visually degraded words. The latter could also signal that control adjustments are needed, because of a lack of bottom-up information. The results showed that both agreement violations and degraded words elicited long-lasting positivities—though with different onsets and some distributional differences. It is proposed that the general process underlying these positivities is similar. Both language errors and degraded words signal the need for adjustments in control to reprocess the input—either to check for errors, or to identify the word. However, depending on the type and complexity of the information that interrupts comprehension, the positivities vary in onset and/or scalp distribution. An unexpected finding was that the ERP pattern to agreement violations was influenced by presentation order. Participants who had seen the syntactic block first showed a P600 effect to agreement violations, while participants who had seen the degradation block first showed an N400 effect. This finding might indicate that different strategies develop to process agreement violations, depending on the context in which they are embedded.