This longitudinal case study describes an anomalous error produced by a child with specific language impairment (SLI), MM, whose language development was documented from ages 3 through 7 years. Twelve spontaneous language samples were analyzed. Across nine language samples MM produced the phonetic sequence heard in the [there/their/they're homonym set] in ungrammatical contexts as if it were the nominative third person plural pronoun. The phonetic sequence used may have been either "their", a case error, or "they're", inclusion of a form of [BE] in an unexpected context. In this paper, we examine and describe the distribution of the [there/their/they're homonym set] and the development of pronominal forms and the verb [BE] in order to account for the anomalous error.