The present study was designed to investigate the electrophysiological consequences of a mismatch between initial phoneme expectations and the actual spoken input. Participants were presented with a word/nonword prompt with the instruction to delete the initial sound (e.g., snap without the /s/; snoth without the /s/) and determine the resulting segment. Following the prompt, an aurally presented response that matched/mismatched expectations (e.g., nap/tap; noth/toth) was presented. The Phonological Mapping Negativity (PMN), a response associated with phonological processing, was largest to mismatching responses, and was not dependent on the lexical status of response items. An N400-like response was also largest to mismatching responses; however, in contrast to the PMN, the N400-like response differentiated mismatching words from mismatching nonwords. These findings highlight a functional dissociation between the PMN and N400, and establish the PMN as a neural marker representing the goodness-of-fit between initial phoneme expectations and the actual spoken input.