cray = C = crayola
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.
--The Jargon File version 4.3.1, ed. ESR, this entry manually entered by rootbeer277.