Lesson Learned: Past successful dam performance does not guarantee future successful performance.

Decades of incident-free operation can create false confidence that dams will remain safe. This article shows why past performance is a poor predictor of future safety through three technical case studies. At Lawn Lake Dam (1982), leakage at a buried outlet works joint likely initiated internal erosion (piping) that led to embankment collapse, compounded by infrequent inspections of a remote site. At Oroville Dam (2017), long-standing leakage, thin slab sections above drains, joints without waterstops, shallow anchors, and weathered foundation zones enabled slab uplift and rapid foundation erosion, even though the project had safely passed higher flows before. At Spencer Dam (2019), an extreme ice-run destroyed the radial gates and limited operator access, producing overtopping and two embankment breaches; earlier ice-damage history was obscured by lost records and staff turnover. Human factors, such as complacency, normalization of abnormal conditions, and eroded institutional memory, help explain how apparent success masks growing risk. The article closes with actions to avoid this false sense of security: update hydrologic and ice-hazard assumptions and design allowances; target inspections and maintenance on known weak points and operability under adverse conditions; preserve records and develop a learning-oriented safety culture; and add redundancy to maintain control when conditions deteriorate.