Diagnostic error remains an ongoing challenge within patient safety. Using claims data, this retrospective study found that among Medicare patients presenting to the emergency department with symptoms consistent with ruptured abdominal aortic aneurysm, acute myocardial infarction (AMI), stroke, aortic dissection, and subarachnoid hemorrhage, between 2.3% (AMI) and 4.5% (aortic dissection) of patients were discharged without a diagnosis.