Diagnostic errors are a known threat to patient safety, and measuring their prevalence is challenging, particularly outside pathology and radiology settings. Past studies have highlighted classification systems and related prevention strategies, including the adoption of checklists. This study explored the use of a situational awareness (SA) framework to understand diagnostic errors in a primary care setting. Investigators interviewed providers involved in a diagnostic error and revealed that one level of SA was lacking (e.g., information perception, information comprehension, forecasting future events, and choosing appropriate action based on the first three levels). The authors found that applying the SA framework to analyze such errors provided deeper insight into the provider–work system interaction, which included important interface with the electronic health record. A past AHRQ WebM&M perspective and interview discussed diagnostic errors in medicine.