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Early in the deployment of the Advanced Simulation and Computing (ASC) Q supercomputer, a higher-than-expected number of single-node failures was observed. The elevated rate of single-node failures was hypothesized to be caused primarily by fatal soft errors, i.e., board-level cache (B-cache) tag (BTAG) parity errors caused by cosmic-ray-induced neutrons that led to node crashes. A series of experiments was undertaken at the Los Alamos Neutron Science Center (LANSCE) to ascertain whether fatal soft errors were indeed the primary cause of the elevated rate of single-node failures. Observed failure data from Q are consistent with the results from some of these experiments. Mitigation strategies have been developed, and scientists successfully use Q for large computations in the presence of fatal soft errors and other single-node failures.