Background: The sleeve gastrectomy is an example of minimally invasive surgery. It is important to determine the critical steps of the procedure in order to reduce complications and increase safety and efficiency. Objective: The aim of this study was to detect the key elements of the sleeve gastrectomy and find the potential hazard zones of the procedure. Setting: Bariatric department of a large teaching hospital in the Netherlands. Methods: A prospective clinical observation study was performed including 60 sleeve gastrectomy procedures. An expert panel determined the key steps, and two experts assessed the procedures systematically for technical errors according to the principles of Observational Clinical Human Reliability Assessment (OCHRA). Results: A total of 213 technical errors have been made, and the majority were made during mobilization of the greater curvature and during stapling of the stomach. In 44.6 %, errors had consequences and 96 additional actions were performed. There was a significant correlation between errors during opening of the lesser sac and postoperative complications, and between repositioning of the stapler and postoperative complications. Conclusions: In this study, the 13 key steps of the SG were defined, and OCHRA was considered a valuable assessment tool for surgical performance and potential hazard zones. Most consequential errors are made during dissection of the greater curvature and during stapling of the stomach. Errors during the start of mobilization of the greater curvature and repositioning of the stapler lead to longer duration of the procedure and are associated with a higher risk of postoperative complications.