In reading, a comma in the wrong place can cause more severe misunderstandings than the lack of a required comma. Here, we used ERPs to demonstrate that a similar effect holds for prosodic boundaries in spoken language. Participants judged the acceptability of temporarily ambiguous English “garden path” sentences whose prosodic boundaries were either in line or in conflict with the actual syntactic structure. Sentences with incongruent boundaries were accepted less than those with missing boundaries and elicited a stronger on-line brain response in ERPs (N400/P600 components). Our results support the notion that mentally deleting an overt prosodic boundary is more costly than postulating a new one and extend previous findings, suggesting an immediate role of prosody in sentence comprehension. Importantly, our study also provides new details on the profile and temporal dynamics of the closure positive shift (CPS), an ERP component assumed to reflect prosodic phrasing in speech and music in real time. We show that the CPS is reliably elicited at the onset of prosodic boundaries in English sentences and is preceded by negative components. Its early onset distinguishes the speech CPS in adults both from prosodic ERP correlates in infants and from the “music CPS” previously reported for trained musicians.