A targeted review of documented waterborne disease outbreaks over the past decades reveals some recurring themes that should be understood by drinking-water suppliers. Evidence indicates the outbreaks are often linked to some significant change in conditions that provides a sudden challenge to a water system. Severe weather events, such as heavy rainfall or runoff from snow melt, as well as treatment process and system changes, are common risk factors for drinking-water outbreaks. Failure to recognise warning signs and complacency are important contributors to drinking water becoming unsafe. Drinking-water suppliers must focus on competence and vigilance in maintaining effective multiple barriers appropriate to the challenges facing the drinking-water system. Understanding the risk factors and failure modes of waterborne disease outbreaks is an essential component for effective management of community drinking-water supplies and ensuring the delivery of safe drinking-water to consumers.