A stuck valve nearly caused America's first full nuclear meltdown

At the Three Mile Island plant in Pennsylvania in 1979, a relief valve stuck open and silently drained cooling water from a reactor core while instruments misled operators into thinking the core had plenty, melting roughly half of it before the mistake was caught. No deaths were directly attributed to the accident, but it remains the most serious commercial nuclear accident in US history and reshaped nuclear safety regulation nationwide.

— US Nuclear Regulatory Commission, Three Mile Island Accident — NRC Backgrounder (historical review)