This conversation analytic paper investigates how speakers self-position or are other-positioned as members of a certain social group (e.g., competent speakers of a language) through other-initiated repair. Findings illustrate the complexity of linguistic membership categories by demonstrating that they continually shift depending on local interactional goals and documenting how shifts are accomplished. The different levels and types of linguistic and cultural knowledge that are invoked in instances of repair on specific lexical items demonstrate the complexity of linguistic membership categorization, and this indicates a need to problematize the use of a priori and overly-vague labels like ‘non-native speaker’. Findings contribute to our understanding of the functions of other-initiated repair and the mechanisms of co-constructing membership (categorization) and thus social identity in interaction. They also raise questions about the relationship between amounts and types of knowledge and further our understanding of the construction of expert/novice categories via knowledge displays and negotiation.