Error Guessing Technique

Error Guessing is an experience-based software testing technique where testers use their intuition, domain knowledge, past experience, and knowledge of common defects to predict where problems are likely to occur.

Instead of relying only on formal test design techniques, the tester asks:

“What could go wrong here?”

It is particularly useful for finding defects that may not be identified through techniques such as Equivalence Partitioning or Boundary Value Analysis.

How Error Guessing Works

A tester considers:

• Common mistakes developers may make.

• Defects found in similar features or previous releases.

• Areas that have historically caused problems.

• Unusual user behavior.

• Weak or complex parts of the application.

• Interactions between different features.

• Unexpected or invalid inputs.

The tester then creates targeted test cases based on these possibilities.

Example: Login Feature

For a login page, an experienced tester may think about:

• Leaving the username empty.

• Leaving the password empty.

• Entering an incorrect password.

• Using an unregistered email.

• Entering a very long username or password.

• Entering leading or trailing spaces.

• Using special characters.

• Clicking the Login button multiple times quickly.

• Refreshing the page during login.

• Trying to log in after the session expires.

• Attempting login after multiple failed attempts.

Some of these scenarios may not be explicitly mentioned in the requirements, but experience tells the tester they are worth checking.

Example: File Upload

Suppose an application allows users to upload profile images.

An error-guessing approach might test:

• Uploading an unsupported file type.

• Uploading a very large file.

• Uploading an empty file.

• Using a very long filename.

• Uploading a file with special characters in its name.

• Uploading the same file multiple times.

• Uploading a corrupted image.

• Cancelling the upload during processing.

• Uploading a file with an unexpected extension.

These tests are based on likely failure points rather than only the documented happy path.

Common Areas for Error Guessing

Experienced testers often pay extra attention to:

• Empty or missing inputs.

• Invalid formats.

• Boundary values.

• Very large or very small inputs.

• Special characters.

• Duplicate data.

• Rapid repeated actions.

• Unexpected navigation.

• Session expiration.

• Network interruptions.

• Browser refresh or back-button behavior.

• File uploads and downloads.

• Permissions and access control.

• Integration points between systems.

Advantages

• Can uncover unexpected defects.

• Uses tester experience and domain knowledge.

• Helps find issues missed by formal techniques.

• Useful when requirements are incomplete.

• Can be applied quickly.

• Works well with exploratory testing.

Limitations

Error guessing depends heavily on the tester's experience and understanding of the application.

Because it is experience-based:

• Different testers may identify different risks.

• Important scenarios may still be missed.

• It should not replace structured test design techniques.

• The quality of results depends on the tester's knowledge and creativity.

Therefore, Error Guessing works best as a complement to structured testing techniques, not as a replacement for them.