Group psychotherapy provides unique opportunities for clinical errors in the selection of patients and composition of therapy groups. This article introduces some of the difficulties and complexities that can be associated with group composition and patient selection errors. Clinical vignettes from psychodynamic/interpersonal psychotherapy groups are used to illustrate three variations of group composition and selection errors. The first vignette depicts an error in selecting a disruptive patient into a fledgling group. The second vignette portrays an unsuccessful integration of a withdrawn, inhibited patient into an active, exploratory group. The third scenario illustrates challenges associated with poor quality of object relations in homogeneous group composition. Although research on group therapy composition and patient selection is limited, relevant empirical literature is integrated in our discussion of clinical implications and recommendations.