A comprehensive view of patient safety hazards requires identifying safety issues through multiple data sources. This Danish study analyzed safety problems in oncology care through voluntary error reports, retrospective chart review using the Global Trigger Tool, and patient reports. While each data source revealed unique hazards, common problems in this patient population included treatment-related harm (from chemotherapy and other procedures), health care–associated infections, and problems related to communication between providers. An AHRQ WebM&M commentary discusses a preventable complication in a patient receiving outpatient chemotherapy.