Understanding Bug, Defect, Fault, Error, and Failure in Software Testing

In software testing, terms like bug, defect, fault, error, and failure are often used interchangeably, but they have distinct meanings. Understanding these differences is crucial for software testers, as interviewers frequently ask about them. So, what sets a bug apart from a defect? How does an error lead to a failure? In this blog post, we’ll break down these key testing terms with clear explanations and real-world examples.

In software testing, these terms are often used interchangeably, but they have distinct meanings:

Bug – A flaw or issue in the software that causes it to behave incorrectly. It is usually found during testing before the product is released.

Defect – A broader term for any flaw in the software that deviates from the expected requirements. A defect can exist in requirements, design, or code. If a defect is found before the software is released, it’s usually called a defect rather than a failure.

Fault – A defect in the code or design that, when executed, may lead to an incorrect state or behavior. A fault is the root cause of an issue in the system.

Error – A mistake made by a developer, tester, or user. Errors in design or code lead to defects or faults in the system.

Failure – The software does not function as expected during execution, leading to incorrect output or behavior. A failure happens when a fault is triggered under specific conditions.

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Example: 

• A developer makes an error in writing code. 
• This introduces a fault in the system. 
• During testing, this is detected as a bug or defect because it causes an incorrect output. 
• If this defect reaches production and affects end users, it results in a failure.

Let’s go through a real-world e-commerce scenario to explain bug, defect, fault, error, and failure in software testing.

Scenario: Online Shopping Cart Issue

1. Error (Human Mistake by Developer)

A developer accidentally writes incorrect logic while implementing the cart update function:

python

total_price = total_price * discount_percentage  
# Wrong: Should be total_price - (total_price * discount_percentage)

• The developer intended to apply a discount, but instead of subtracting the discount, the code multiplies the total price by the discount percentage, leading to incorrect calculations.

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2. Fault (Defective Code Exists in the System)

• This incorrect discount calculation logic is a fault in the system.
• If this code is executed, it will cause incorrect total price calculations for users.

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3. Defect/Bug (Found During Testing)

• A tester notices that when a 10% discount is applied on a $100 order, the total price is shown as $10 instead of $90.
• The tester reports it as a defect (or bug) in the tracking system.
• Example Bug Report: "Discount calculation is incorrect: The total price is reduced too much when a discount is applied."

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4. Failure (Issue in Production Affects Users)

• If this defect is not fixed and the software is deployed, customers will see incorrect prices when checking out.
• A customer adds a $500 product with a 20% discount, expecting to pay $400, but the system shows $100 instead.
• This results in business losses and customer complaints.

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Summary Table

Term	Meaning	E-commerce Example
Error	Human mistake in code/design	Developer writes incorrect discount formula
Fault	Defective code that may cause issues	Incorrect discount logic exists in the system
Defect/Bug	Issue found during testing	Tester notices incorrect pricing in the cart
Failure	Issue affects users in production	Customers see wrong prices and checkout incorrectly

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This example shows how an error by a developer introduces a fault in the system, which is detected as a bug/defect in testing. If left unfixed, it results in a failure that negatively impacts customers and the business.

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