Regression testing

Regression testing verifies that things which used to work still work. A regression is a defect in existing behaviour introduced by a new change — the most demoralising category of bug, because it appears in a part of the system nobody touched deliberately.

Doing it manually does not scale: the surface to check grows with every release while the time available does not, so coverage silently shrinks to whatever the tester can manage on the day. Automating it is what allows a system to keep changing safely as it gets larger.

The pragmatic rule is that every bug fixed in production earns a test. It costs very little at that moment, the reproduction is already understood, and it guarantees that particular failure cannot return unnoticed.