Introduction Sharing mental models is essential for high-performance teams, and speaking up is key for exchanging critical insights, especially during medical errors. Understanding how health providers and trainees voice their concerns is crucial for improving speaking-up behavior. This study aims to fill a gap in the literature by examining how medical students speak up when they encounter medical errors and assessing the impact of training on their speaking-up patterns. Method A quasi-experimental study involving 146 students, who were divided into two groups, was conducted in Northern Taiwan. One group of students encountered life-threatening scenario before intervention, followed by a faculty-led personalized debriefing session, then a non-life-threatening scenario after the intervention. Another group of students underwent these sessions in the reverse order. Students’ Speaking-up patterns, including expression style, form and attitude, and their speaking-up confidence were assessed at pre- and post-intervention scenarios. Results During pre-intervention scenario, in expression style, 50 students (34.5%) addressed their concerns to medical errors with direct expression and 14 students (9.7%) utilized indirect hint to express their concerns. In expression form, 31 students (21.4%) addressed their concerns to medical errors with affirmative sentences and 33 students (22.8%) asked questions to express their concerns. In speaking-up attitude, 47 students (32.4%) used unoffensive words, while 17 students (11.7%) used offensive words. After intervention, significantly change of speaking-up styles, forms, and attitude were observed along with their speaking-up confidence (p < 0.001). Discussion Medical students are inclined to speak up in the event of medical errors using more direct expression and affirmative sentences, along with increased speaking-up confidence after simulation scenario learning and faculty-led personalized debriefing. Healthcare educators can focus more on discussing with students the advantages and disadvantages of various approaches of speaking-up in medical errors, helping them to develop effective speaking-up behaviors in a variety of medical contexts.