Context: Inadequate documentation of observed trainee incompetence persists despite research-informed solutions targeting this failure to fail phenomenon. Documentation could be impeded if assessment language is misaligned with how supervisors conceptualise incompetence. Because frameworks tend to itemise competence as well as being vague about incompetence, assessment design may be improved by better understanding and describing of how supervisors experience being confronted with a potentially incompetent trainee. Methods: Following constructivist grounded theory methodology, analysis using a constant comparison approach was iterative and informed data collection. We interviewed 22 physicians about their experiences supervising trainees who demonstrate incompetence; we quickly found that they bristled at the term 'incompetence,' so we began to use 'underperformance' in its place. Results: Physicians began with a belief and an expectation: all trainees should be capable of learning and progressing by applying what they learn to subsequent clinical experiences. Underperformance was therefore unexpected and evoked disbelief in supervisors, who sought alternate explanations for the surprising evidence. Supervisors conceptualised underperformance as: an inability to engage with learning due to illness, a life event or learning disorders, so that progression was stalled, or an unwillingness to engage with learning due to lack of interest, insight or humility. Conclusion: Physicians conceptualise underperformance as problematic progression due to insufficient engagement with learning that is unresponsive to intensified supervision. Although failure to fail tends to be framed as a reluctance to document underperformance, the prior phase of disbelief prevents confident documentation of performance and delays identification of underperformance. The findings offer further insight and possible new solutions to address under-documentation of underperformance. | 1149 GINGERICH Et al.