When I was peripherally working on some HPC stuff, there was a comment by one of the hardware guys that it mattered which national lab you were building the supercomputer for, because the guys at high altitude like Los Alamos get a lot more bitflips than someone closer to sea-level like Argonne. Although that said, for an exascale supercomputer, the mean time between uncorrected bit flip somewhere in the machine is on the order of a few hours, which means that large supercomputer-scale workloads should actually expect to hit a bit flip in their computation.