Simulation with simulated patients is increasingly used in occupational therapy. Ensuring the debrief component meets student needs to develop their self-assessment skills for participating in a self-regulating profession is essential. This explanatory mixed-methods research sought to explore factors contributing to accurate and inaccurate self-assessment of simulation performance for novice occupational therapy learners in a part-time introductory fieldwork course. Self-ratings and preceptor-ratings of performance on eleven simulation objectives were compared for sixty-five novice occupational therapy students. Factor analysis was used to explore contributors to differences in ratings between students and preceptors. Students' written plus-delta debrief reflections were analyzed to explore what evidence they used to self-assess performance and their remaining questions not addressed with self-debrief. There was a significant difference in the rating scores between students and preceptors for all objectives (p<0.05). Students rated themselves on average higher than preceptors and they often missed safety concerns noted by the preceptors. Factor analysis indicated that the type of learning objective contributed to rating difference with objectives related to communication differing from those related to demonstration of skills. Deductive qualitative content analysis of reflections indicated that students give significant weight to simulated patients' agreeability, willingness to participate, and reported comfort as evidence of success when reflecting on simulations, and rarely use best-practice guidelines, theories, or principles to self-assess their performance. Novice students may need guidance and explicit training on what diverse types of evidence they may use to support self-assessment and reflection on performance in a simulation for different types of learning objectives.