Answer.

Manual testing of business logic is aimed at verifying the compliance of the implemented functions of the application with the business requirements and use cases described by the client or analysts.

Background

With the development of IT products, the complexity of business logic has increased. Applications began to include branching scenarios, conditions, and exceptions, and automated testing does not always cover unique user stories. Manual testing allows for "trying on" the necessary logic to real client tasks.

Problem

In most cases, the pitfalls are that the tester:

relies only on documentation, ignoring real user scenarios;
does not cover all exceptions;
misses complex dependencies between business rules.

Solution

For quality manual testing of business logic, one should:

dissect, clarify, and discuss business requirements with analysts/client;
build user scenarios (user stories), paying attention to valid and invalid combinations of input data;
check boundary and exceptional situations within business processes;
document not only bugs but also overlooked requirements or inaccuracies.

Key features:

Attention to detail: even a minor inaccuracy in business logic can lead to significant losses.
Interactive communication with the client: it is important to receive feedback on contentious points.
Coverage of all alternative paths: it is necessary to test not only standard but also non-standard scenarios.

Tricky Questions.

Can one completely rely on test documentation and requirements when testing business logic?

No. Often, the documentation does not cover all aspects of the application's behavior, especially in complex branching scenarios. Additionally, it is important to clarify details with requirements owners and explore the system through exploratory testing.

Is it mandatory to test all possible negative scenarios of business logic?

Yes, testing only "correct" (positive) scenarios leads to missing critical bugs that occur on incorrect inputs, user errors, or when business rules are violated.

Is formal confirmation of test steps sufficient to assert that the business logic is implemented correctly?

No. Formal execution of test cases does not guarantee that all business logic functions correctly — it is important to check relationships between conditions and scenarios, evaluate user experience, and ensure alignment with actual business expectations.

Common mistakes and anti-patterns

Focusing solely on requirements without communication with the business
Insufficient coverage of negative scenarios
Ignoring non-standard or overlapping conditions

Real-life example

Negative case

The tester strictly followed the documentation and did not clarify details with the client. He tested only the main activation scenarios in the banking application.

Pros:

Quick coverage of release requirements
Clear testing trajectory

Cons:

Errors not discovered when activating the service at night and with invalid client statuses
Task returned for rework after issues were discovered in production

Positive case

The tester actively interacted with the business analyst, testing not only all formal scenarios but also reference cases with edge conditions (e.g., service unavailability on weekends).