Although supportive-expressive (SE) psychotherapy is one of the most studied psychodynamic therapies today, little is known empirically about effective strategies in SE supervision, or in psychodynamic supervision in general (Diener & Mesrie, 2015; Watkins, 2011). One of the important questions in SE psychotherapy is how to decide when to use supportive and when to use expressive interventions. As a parallel process, this type of decision is relevant also to SE supervision. The present case study focuses on the decision-making process in an SE supervision session: when should supervisors use supportive as opposed to expressive strategies with their supervisees? Our aim is to develop decision rules that reliably support supervisors’ decisions. We analyze a clinical error made by supervisors in this type of decision making, and show how mistakes of this type can either be avoided or, when they occur, how to turn them into opportunities for learning and for the formation of new understanding and growth. Similarly to the finding that therapists with better skills in managing their countertransference feelings were shown to have better outcomes with their patients (Gelso, Latts, Gomez, & Fassinger, 2002; Hayes, Gelso, & Hummel, 2011), we suggest that the management of the supervisors’ feelings, and working through their mistakes with the therapists, can contribute to the supervisory relationship and to the development of the psychodynamic therapists’ skills.