Behavioral and electrophysiological studies have uncovered different patterns of constraint effects on the processing of words in sentences. Whereas response time measures have indicated a reduced scope of facilitation from strongly constraining contexts, event-related brain potential (ERP) measures have instead revealed enhanced facilitation for semantically related endings in such sentences. Given this disparity, and the concomitant possibility of functionally separable stages of context effects, the current study jointly examined expectancy (cloze probability) and constraint effects on the ERP response to words. Expected and unexpected (but plausible) words completed strongly and weakly constraining sentences; unexpected items were matched for contextual fit across the two levels of constraint and were semantically unrelated to the most expected endings. N400 amplitudes were graded by expectancy but unaffected by constraint and seemed to index the benefit of contextual information. However, a later effect, in the form of increased frontal positivity from 500 to 900 ms post-stimulus-onset, indicated a possible cost associated with the processing of unexpected words in strongly constraining contexts.