Turning Fixed Bugs Into Regression Checks
Bugs have a habit of returning. A fix is made, the problem disappears and months later a different change brings it back. Nobody remembers the original fix, so the bug is investigated from scratch, sometimes by a different person who has to learn everything again.
The cure is simple to describe: when a bug is fixed, turn the fix into a check that would fail if the bug returned. Here is how we approach it and what else we learn along the way.
Reproduce first
Before fixing a bug, we try to reproduce it: a specific set of steps or inputs that makes it happen. A reproduction proves we understand the problem rather than guessing at it, and it becomes the starting point for the regression check.
Some bugs are hard to reproduce, especially those involving timing, real customer data or outside services. In those cases, we record what we know and look for the conditions under which the bug appears, rather than applying a speculative fix and hoping.
Fix the cause, not the symptom
A bug often appears in one place and lives in another. A page shows the wrong website's menu, but the cause is a cache key that forgot to include the website. A form loses its contents, but the cause is the page reloading on submission. Fixing the symptom where it appeared leaves the cause free to produce new symptoms elsewhere.
We look for the shared cause and fix it there, which often fixes several bugs at once.
Write the check
Once the cause is fixed, the regression check describes the bug in a way a machine can test. That might be an automated test that recreates the faulty conditions, a rule in an automated content checker or a step in a browser checklist. The check should fail on the old code and pass on the new code.
Where an automated test is not practical, we still record the bug and its fix in the module's documentation, so a future reader knows the trap exists.
Ask where else it lives
Many bugs are examples of a pattern. A missing permission check in one action suggests checking similar actions. A dropdown that lists users from every account suggests checking every dropdown that lists people. After fixing a bug, we ask whether the same mistake could exist elsewhere and look.
That habit is how isolated fixes become platform improvements. Some of our engineering rules, such as building people lists from account memberships, came from exactly this kind of review, which we describe in why we never load every user into a dropdown.
Turn lessons into rules
Some bugs teach a lesson worth writing into our standards. Theme styles that overrode page builder styles taught us the cascade layer rule. A sign-in path that skipped two-factor authentication taught us to route every sign-in through one gate. Those lessons now live in our documentation and in the instructions our assistants read, so they shape new code before it is written.
Keep the fix small
A bug fix is not the moment for a redesign. Large changes bundled with a fix make it harder to review, harder to verify and harder to undo if the fix causes a new problem. We keep fixes focused on the cause, ship them on their own and leave broader improvements for separate, deliberate work. A small fix also makes its regression check easy to understand: one problem, one change, one check.
Be honest about what was fixed
When we report a fix, we say what was fixed, what was tested and what was not. If a fix addresses the most likely cause but the bug could not be reproduced, we say so. If a related problem remains, we say that too. Overstating a fix only leads to surprise when the bug returns.
The compounding benefit
Each regression check is small. Together, they form a growing safety net that makes every future change a little safer. Teams that skip this step keep paying for the same bugs. Teams that do it find that old problems stay solved. It is part of testing what matters most.
Frequently asked questions
What is a regression test?
A regression test checks that a previously fixed bug has not returned. It fails if the old problem reappears.
Should every bug fix include a test?
Where practical, yes. When an automated test is not practical, record the bug and fix in documentation so the trap is known.
Why look for similar bugs after a fix?
Bugs often follow patterns. Checking for the same mistake elsewhere turns one fix into a broader improvement.
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