Answer.

The strategy for choosing between manual and automated testing is determined by many factors, including cost, goals, stability of functionality, and available workforce resources.

Background: In the early days of automation, it was believed that the goal should be to achieve 100% test coverage through automation. Over time, it became clear that not every check should be automated — some tasks are easier, faster, and cheaper to perform manually.

Problem: Automation is only beneficial when the same scenarios are run frequently and repetitively. One-time, rapidly changing, or subjective (visual, UX) checks are much easier to perform manually. The attempt to "automate everything" can quickly inflate the budget, and the benefits may be lost.

Solution: First and foremost, it is advisable to automate what:

Is performed frequently
Is stable in implementation
Is critical to the business

Manual testing is relevant for:

Creative, exploratory scenarios
One-time or rapidly aging tasks
Checking elements that are inconvenient to automate (drag & drop, animations)

Key features:

ROI (return on investment) analysis for each class of tasks
Involvement of QA experts when forming the strategy
Prioritization of critical and regression functionality

Trick Questions.

Is it beneficial to automate all tests in a project?

No, because many tests will change before they can recoup the efforts invested, and maintaining automation will cost more than performing the same tests manually.

Can manual testing be mandatory during a production release?

Yes, because some tasks (especially those related to UI/UX and visual regression) are difficult or impossible to automate adequately.

Is it worth automating tests for "one-off"/exploratory tasks?

No, automation takes time and resources that will not be recouped with a one-time or rarely used test.

Common Mistakes and Anti-Patterns

Automating "everything indiscriminately",
Ignoring the opinions of manual testers,
Lack of cost and benefit analysis (ROI)

Real-life Example

Negative Case

The team decided to automate all manual test cases, including cross-browser testing on non-standard devices and visual layout checks. As a result, hundreds of hours were spent maintaining outdated tests over several months.

Pros:

High percentage of coverage with automated tests

Cons:

Increased maintenance costs
Complexity of releases
Some bugs slip through

Positive Case

The team, along with the manager, formed a list of scenarios critical to the business (registration, payment verification) that were stably automated; everything else is tested manually or partially (using exploratory testing).