In engineering, a corner case (or pathological case) involves a problem or situation that occurs only outside normal operating parameters—specifically one that manifests itself when multiple environmental variables or conditions are simultaneously at extreme levels, even though each parameter is within the specified range for that parameter.
Or a computer server may be unreliable, but only with the maximum complement of 64 processors, 512 GB of memory, and 10,000 signed-on users.
Corner cases are often harder and more expensive to reproduce, test, and optimize because they require maximal configurations in multiple dimensions.
The term "corner case" comes about by physical analogy with "edge case" as an extension of the "flight envelope" metaphor to a set of testing conditions whose boundaries are determined by the 2n combinations of extreme (minimum and maximum) values for the number n of variables being tested, i.e., the total parameter space for those variables.
Where an edge case involves pushing one variable to a minimum or maximum, putting users at the "edge" of the configuration space, a corner case involves doing so with multiple variables, which would put users at a "corner" of a multidimensional configuration space.