Edge Case Matrix

Core rule

Derive tests from input dimensions, not from whatever examples happen to be in front of you.

Minimum matrix

For each relevant boundary, consider: normal, empty, missing/null, one item, many items, minimum/maximum, invalid type, malformed syntax, duplicate values, reordered values, spaces/Unicode, old/new version, permission failure, timeout, partial state, and rerun.

Use pairwise combinations when dimensions multiply; reserve full Cartesian coverage for small high-risk state spaces.

For stateful operations, include interruption between steps and recovery from partially written state. For parsers, include unknown future fields. For installers, include already-installed and broken-installed cases.

The goal is not maximal test count. It is to identify qualitatively different failure classes before users do.