When a crew of three cosmonauts concluded a pioneering 24-day mission aboard Earth’s first space station, an entire nation waited to welcome them. But joy turned to grief after recovery teams opened the descent capsule and discovered all three cosmonauts dead in their seats. A valve meant to equalize cabin pressure with Earth’s atmosphere moments prior to landing had been forced open prematurely to vacuum when the descent module separated explosively from the rest of Soyuz-11, suffocating the crew. Events leading to the depressurization show how unplanned mechanical shock led to single-point failure of a critical assembly, and how complex systems can defeat attempts to ensure comprehensive human understanding of a project’s design from concept development to operation.