brittle

brittle adj.  Said of software that is functional but easily
   broken by changes in operating environment or configuration, or by
   any minor tweak to the software itself.  Also, any system that
   responds inappropriately and disastrously to abnormal but expected
   external stimuli; e.g., a file system that is usually totally
   scrambled by a power failure is said to be brittle.  This term is
   often used to describe the results of a research effort that were
   never intended to be robust, but it can be applied to commercially
   developed software, which displays the quality far more often than
   it ought to.  Oppose {robust}.