regression

When a change breaks something that used to work — a new failure in previously passing tests.

A regression is backsliding: code that passed its tests before now fails after a change, meaning the change introduced a bug or reintroduced an old one. 'Zero regressions' is the reassuring claim that a change broke nothing that previously worked. Tests are often written specifically to 'catch a regression' so the same breakage can't slip through again.