Answer.

Background:

Manual testing originally relied on the habit of testing only those scenarios that met the requirements and expected behavior of the system (so-called "positive scenarios"). Over time, it became clear that software often fails under unexpected or erroneous conditions that were not anticipated.

Problem:

Only positive scenarios do not guarantee the stability and reliability of the application. If negative scenarios (for example, incorrect input, invalid actions) are not tested, serious defects that may arise with real users can be overlooked.

Solution:

Conduct both types of testing:

Positive testing checks whether the system works according to requirements with correct data and user actions.
Negative testing checks the system for stability and robustness against erroneous, non-standard, or incorrect actions and data.

Key features:

It is impossible to ensure quality with only one type of testing.
Negative testing helps find errors that are less frequently encountered during regular use.
Positive cases are based on requirements, negative ones are based on their violation.

Trick questions.

Can negative testing be neglected if the product passes a complete set of positive scenarios?

No. Errors occurring in negative scenarios often have a critical impact on the safety and reliability of the product.

Must negative tests necessarily lead to program errors?

No, a well-implemented program should correctly handle erroneous data in negative scenarios, not "crash" and not produce incorrect results.

Is it equally important to write positive and negative tests for all parts of the system?

No, sometimes for non-critical or established parts of systems, the volume of negative scenarios can be reduced, but for vulnerable and critical areas, it is a necessity.

Common mistakes and anti-patterns

Complete neglect of negative scenarios.
Too superficial negative testing (for example, only one incorrect input was checked).
Misunderstanding the difference between positive and negative testing.

Real-life example

Negative case

In the company testing the registration form on the website, only correct values were checked (valid email, passwords, etc.), ignoring erroneous options.

Pros:

Fast development and testing.

Cons:

After launch, users were able to submit the form with invalid emails, leading to mailing failures and negative reviews.

Positive case

The tester added tests for entering invalid emails, passwords that were too short and too long, and special characters in all fields.