Assessment of trainee performance in the workplace is critical to ensuring high standards of clinical care. However, some supervisors find the task to be challenging, and may feel unable to deliver their true judgement on a trainee’s performance. They may ‘keep MUM’ (that is, keep mum about undesirable messages) and fail to fail an underperforming trainee. In this study, we explore the effect of discomfort on assessors. Methods Using a survey method, supervisors of trainees in the Australasian College of Dermatologists were asked to self-report experiences of discomfort in various aspects of trainee workplace assessment and for their engagement in MUM behaviours including failure to fail. Results Sixty-one responses were received from 135 eligible assessors. 12.5% of assessors self-reported they had failed to fail a trainee and 18% admitted they had grade inflated a trainee’s score on a clinical performance assessment in the previous 12-month period. Assessors who reported higher levels of discomfort in the clinical performance assessment context were significantly more likely to report previously failing to fail a trainee. The study did not reveal significant associations with assessor demographics and self-reports of discomfort or MUM behaviours. Conclusions This study reveals the impact of assessor discomfort on the accuracy of assessment information and feedback to trainees, including as a contributing factor to the failure to fail phenomenon. Addressing assessor experience of discomfort offers one opportunity to impact on the complex and multifactorial issue that failure to fail represents.