Issues are often used—sometimes interchangeably—but they do have nuanced differences.

🐞 Bug

A bug is a flaw or fault in the software that causes it to behave unexpectedly or incorrectly.

It is usually discovered during testing or after the software has been deployed.

In practice, “bug” and “defect” are often used interchangeably, though some distinguish bugs as defects that make it past development into testing or production.

Example: Clicking a “Submit” button doesn’t do anything due to a missing function call.

❌ Defect

A defect is a deviation from the expected behavior, usually identified during development or testing.

It is a more formal or internal term used to describe something that doesn’t meet requirements or design specifications.

All bugs are defects, but not all defects are necessarily user-facing bugs.

Example: A login page allows submission with a blank password, contrary to specifications.

🧠 Error

An error is a human action that produces an incorrect result—typically a mistake made while coding, designing, or interpreting requirements.

Errors lead to the introduction of defects in the code or system.

Example: A developer miscalculates a formula due to misunderstanding the logic.

💥 Failure

A failure is when the software does not perform its intended function at runtime.

Failures are the observable results of defects or bugs when the software is executed.

Example: The application crashes when uploading a file over 2MB.

📄 Issue

An issue is a broad and generic term, often used in project management and bug tracking tools like JIRA or GitHub.

It can refer to bugs, feature requests, tasks, improvements, or questions.

Not all issues are defects or bugs.

Example: “Login page issue” may refer to a visual enhancement, a bug, or a request to change behavior.

⚙️ Relationship Between Terms:

Error (human mistake) ➡ Defect (in code or design) ➡ Bug (detected by testing or users) ➡ Failure (observable problem during execution).