In the language domain, most studies of error monitoring have been devoted to language production. However, in language perception, errors are made as well and we are able to detect them. According to the monitoring theory of language perception, a strong conflict between what is expected and what is observed triggers reanalysis to check for possible perceptual errors, a process reflected by the P600. This is at variance with the dominant view that the P600 reflects syntactic reanalysis or repair, after syntactic violations or ambiguity. In the present study, the prediction of the monitoring theory of language perception was tested, that only a strong conflict between expectancies triggers reanalysis to check for possible perceptual errors, reflected by the P600. Therefore, we manipulated plausibility, and hypothesized that when a critical noun is mildly implausible in the given sentence (e.g., "The eye consisting of among other things a pupil, iris, and eyebrow..."), a mild conflict arises between the expected and unexpected event; integration difficulties arise due to the unexpectedness but they are resolved successfully, thereby eliciting an N400 effect. When the noun is deeply implausible however (e.g., "The eye consisting of among other things a pupil, iris, and sticker..."), a strong conflict arises; integration fails and reanalysis is triggered, eliciting a P600 effect. Our hypothesis was confirmed; only when the conflict between the expected and unexpected event is strong enough, reanalysis is triggered.