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The Free On-line Dictionary of Computing (foldoc)

1. [scientific computation] Used of a data set that is grossly
atypical of normal expected input, especially one that exposes
a weakness or bug in whatever algorithm one is using. An
algorithm that can be broken by pathological inputs may still
be useful if such inputs are very unlikely to occur in
practice.

2. When used of test input, implies that it was purposefully
engineered as a worst case. The implication in both senses is
that the data is spectacularly ill-conditioned or that someone
had to explicitly set out to break the algorithm in order to
come up with such a crazy example.

3. Also said of an unlikely collection of circumstances. "If
the network is down and comes up halfway through the execution
of that command by root, the system may just crash." "Yes,
but that's a pathological case." Often used to dismiss the
case from discussion, with the implication that the
consequences are acceptable, since they will happen so
infrequently (if at all) that it doesn't seem worth going to
the extra trouble to handle that case (see sense 1).

[{Jargon File}]