Speech production is an extremely rapid and seemingly effortless process with speech errors in normal subjects being rare. Although psycholinguistic models incorporate elaborate monitoring mechanisms to prevent and correct errors, the brain regions involved in their commitment, detection, and correction have remained elusive. Using event-related brain potentials in a task known to elicit spoonerisms representing a special class of sound errors, we show specific brain activity prior to the vocalization of such spoonerisms. Source modeling localized this activity to the supplementary motor area in medial frontal cortex. We propose that this activity reflects the simultaneous activation of 2 competing speech plans on processing levels related to the construction of a rather "phonetic" speech plan contrasting with the traditional view, assuming the substitution of abstract phonological representations as the main source for sound errors.