Retrospective: A Regression That Slipped Through

The release changed a single line in the export feature. After deployment the client reported wrong tax rates on the invoice page again. We had fixed that bug three months earlier; the fix was overwritten and no regression test covered it.

Three failures in the chain

While editing export logic, the developer did not notice it reused a calculation function from invoicing.

Regression cases covered only the changed area, not downstream features sharing the logic.

Review looked at the diff without checking who else calls that function.

Improvements

Every fixed defect gets an automated case tagged for the regression suite, run on every release.

Review requires an impact check: list the callers of modified functions and confirm each.

Shared utilities carry comments describing their usage so they are not edited casually.

Regressions are rarely untested; they are unknown. Analysing impact as a habit beats adding more tests.