Bug (Defect)
A flaw in software that causes it to behave differently from its specification or user expectation.
Full definition
A bug (also called a defect, fault, or issue) is any deviation between expected and actual behavior of software. Bugs can range from cosmetic issues (a misaligned button) to critical failures (data loss, security vulnerabilities, system crashes).
A well-written bug report includes: a clear title, steps to reproduce, expected vs actual result, environment details (OS, browser, device), severity and priority, and supporting evidence (screenshots, logs, video).
Bug lifecycle typically follows: New → Assigned → In Progress → Fixed → Verified → Closed. Understanding this lifecycle is essential for any QA role.
Common bug classifications by severity:
- Blocker: System crash, data loss, no workaround
- Critical: Major feature broken, workaround exists
- Major: Feature works but with significant issues
- Minor: Cosmetic or low-impact issues
- Trivial: Typos, minor UI polish
Examples
- 1.Login button does nothing when clicked (Critical)
- 2.Price shows $0.00 instead of actual amount (Blocker)
- 3.Username field accepts 10,000 characters (Major)
Interview tip
Be ready to write a bug report on the spot. Practice with real apps — find a bug on any website and write it up with proper format.