This case study examines the reasoning of a clinical supervisor as she assesses preservice teacher candidates with a state-mandated performance assessment instrument. The supervisor's evaluations were recorded using video annotation software, which allowed her to record her observations in real-time. The study reveals some of the inherent challenges in clinical supervision and the use of a state's mandated performance rubrics to evaluate teacher competencies. Findings indicate that the clinical supervisor found it difficult to interpret rubric criteria, often made tenuous claims about candidates' performance, and tended to require students to design lessons that were artificial demonstrations of mandated competencies. Findings also suggest that the difficulties faced by the clinical supervisor were likely connected to inadequate professional development regarding the use of the state-mandated performance assessment instrument. The article concludes with a discussion of the need for better professional development for clinical supervisors given their important role in the professional development of tomorrow's teachers and suggests other areas for future research. (Contains 3 tables.)