Answer.

Regression manual testing is the process of rechecking already tested functions of a system after changes have been made to the code, to ensure that these changes have not introduced new defects in the stable parts of the product.

Background: At the beginning of the software lifecycle, testers usually focus on checking new features. Over time, the system undergoes changes, increasing the risk of unintended regressions. Many errors in software history have arisen after a fix that "broke" something that was previously working, which is why regression testing has gradually become a mandatory practice.

Problem: The main dilemma is how to effectively and promptly review the maximum number of functions amidst constant changes. Approaching the task chaotically or inconsistently can lead to missing critical defects, exceeding deadlines, overloading the team, and sometimes the checks becoming merely formalities.

Solution: To effectively organize regression testing, one should:

Have a set of stable, maintained test cases or checklists for key functions;
Regularly update these sets as improvements are made;
Automate routine tasks where possible, and select critical or non-standard scenarios for manual testing;
Agree on the priority of functions for testing based on business risks and frequency of changes.

Key Features:

Regression not only identifies obvious bugs but also hidden chains of errors related to module interactions.
It is important to regularly review and prioritize test case sets.
The optimal strategy is to combine manual checks with automated tests where possible.

Tricky Questions.

At what stage is it best to start regression testing — after all changes or parallel with their implementation?

Many mistakenly believe that regression is conducted only after all changes are completed. In reality, it is more efficient to plan and partially perform it as changes are made to respond quickly to critical failures.

Is it sufficient to write a regression test case once and never update it again?

No, regression test cases require constant updating: as business logic or interfaces change, the system of tests should change alongside the product.

Is smoke testing part of regression?

No, smoke testing is a quick elimination of obvious failures after a build, while regression covers a broader functionality and searches for failures "deeper."

Common Mistakes and Anti-Patterns

Outdated test cases — these render testing a useless formality.
Ignoring less noticeable but important scenarios.
Lack of clear prioritization — giving the same attention to both key and trivial functions.

Example from Life

Negative Case

The team releases a version, the tester formally checks features against an outdated list of test cases, unaware of the latest API modification. An old bug is fixed, but a critical defect appears in another part of the system, which no one noticed before the launch.

Pros:

Time savings on working through test cases.

Cons:

High risk of missing important regressions.
Decreased trust in testing quality.

Positive Case

Before the release, testers updated the regression checklists, discussed the current risk areas with analysts, prioritized tests based on real scenarios, and conducted manual regression on key flows.