We investigate interpersonal difficulties that student teachers and cooperating teachers may experience during the teaching internship by exploring the tension between one high school mathematics student teacher and his cooperating teacher. We identified seven causes of this tension, which included different ideas about what mathematics should be taught, how it should be taught, and a strained personal relationship. We compare these findings with results from interviews with six other student teachers and eight of their mentors to explore the uniqueness of this case. We also offer suggestions for better preparing student teachers and cooperating teachers for the teacher internship.