Background. Despite the emphasis on medical error as a major cause of hospital morbidity and mortality, there has been little published work on errors committed by trainees. This issue is particularly relevant to the training of surgeons and was addressed by our study. Methods. Sixty simulated laparoscopic cholecystectomies performed on restructured pig tissue models by 60 surgical trainees provided the study material. The unedited videotapes were analyzed by observational HRA of the component steps of the procedures. Ten generic forms of observable error types were used to categorize patterns of failure. Error probabilities with specific instruments were also calculated. Results. A total of 1067 errors were identified by observational HRA: 331 consequential and 736 without consequence (ie, total error rate of 18 [SD ± 10]) per procedure. The study documented a wide variation in the number of errors between the 60 trainee surgeons. The important underlying factors for the trainee errors were (1) omission of important steps, (2) execution of steps in the wrong sequence, and (3) use of excessive force. These 3 errors accounted for 92% of consequential errors. Conclusions. This study has shown that trainees vary considerably in their propensity to commit errors. This variability indicates that the surgical training in component skills for laparoscopic surgery should be flexible and individualized. Three mechanisms account for the majority of errors and indicate that skills training in surgery has to be structured, menu driven, and tailored to individual needs.