cray instability n.
1. A shortcoming of a program or algorithm that manifests
itself only when a large problem is being run on a powerful
machine (see cray). Generally more
subtle than bugs that can be detected in smaller problems running
on a workstation or mini. 2. More specifically, a shortcoming of
algorithms which are well behaved when run on gentle floating
point hardware (such as IEEE-standard or PDP-series machines) but
which break down badly when exposed to a Cray's unique `rounding'
rules.