A corner case is a problem or situation that occurs only outside of normal operating parameters--especially one that manifests itself when multiple environmental variables or conditions are simultaneously at extreme levels.
For example, a stereo speaker might distort audio, but only when played at maximum volume, maximum bass, and in a high-humidity environment. Or a computer server may be unreliable, but only with the maximum complement of 64 processors, 512 GB of memory, and over 10,000 signed-on users.
Contrast a corner case with an edge case, an issue that occurs only at a (single) maximum or minimum parameter. For example, a speaker that distorts audio at maximum volume, even in the absence of other extreme settings or conditions.
Corner cases are part of an engineer's lexicon--especially an engineer involved in testing or debugging a complex system. Corner cases are often harder and more expensive to reproduce, test, and optimize because they require maximal configurations in multiple dimensions. They are frequently less-tested, given the belief that few product users will, in practice, exercise the product at multiple simultaneous maximum settings. Leading-edge users therefore routinely find corner case errors. There are cases where apparently impossible cases have occurred; one such case, that of the Therac 25 radiation therapy machine, caused deaths.
See also: edge case