Most Common Types of Bugs in Software Testing

11 Types of Bugs in Software Testing

Bugs can take many forms in software testing, each of which affects an application in a different way. Recognizing the different types of bugs is important for testers and developers alike, so they can identify and fix the issues.

1. Functional Bugs

Functional bugs cause deviations in the expected behavior of features in a software application as defined by requirements or specifications. They occur when a feature does not perform its intended task, directly affecting the user experience. Functional bugs are among the most common and immediately noticeable defects in software applications.

These bugs break the expected outcomes for users. For example, a user interacting with a button, form, or workflow in a software application expects specific results; functional defects cause these elements to behave differently than intended, leading to errors, blocked processes, or incorrect outputs.

Examples:

• Authentication issues: Login forms rejecting valid credentials or allowing access with invalid ones.
• eCommerce workflow errors: “Add to Cart” or “Checkout” features failing to update the cart or complete transactions correctly.
• Search and filtering issues: Search returns no results or incorrect results despite valid input queries.
• Form submission errors: Contact forms, registration forms, or surveys failing to submit data or showing incorrect validation messages.

You can perform functional testing to check that every feature behaves exactly as specified in the requirements; verify all input combinations, workflows, and edge cases so nothing breaks unexpectedly.

2. Usability Bugs

Usability bugs occur when a software application functions technically but is difficult, confusing, or inefficient for users to interact with. These defects arise from design or interface flaws such as unclear navigation, inconsistent elements, poor readability, low contrast, tiny buttons, or broken accessibility features.

They prevent users from performing tasks smoothly, reducing productivity and satisfaction. Unlike functional bugs, the software may “work,” but usability bugs frustrate users, hinder adoption, and can lead to software abandonment if not addressed through careful UX testing and validation.

• Poor navigation: Users struggle to locate key features due to confusing menus or layouts.
• Inconsistent interface elements: Buttons, labels, or icons behave differently across screens or sections.
• Accessibility issues: Features fail to support users with impairments, such as missing screen reader compatibility.
• Mobile responsiveness issues: Interfaces do not adapt properly across devices, causing layout or interaction issues.
• Unreadable text or insufficient contrast: Information is hard to read, leading to misinterpretation or missed actions.

For more patterns you’re likely to encounter, see this overview of common UI bugs.

Also, perform usability testing by walking through the interface as a user would; focus on navigation, clarity, and error messages to make the experience smooth and intuitive.

3. Compatibility Bugs

Compatibility bugs occur when a software application functions correctly in one environment but fails or behaves inconsistently in others. These defects arise due to differences in operating systems, browsers, devices, screen sizes, resolutions, or hardware configurations.

For example, a feature may work in Chrome but break in Safari, or a mobile app may crash on older Android versions while running smoothly on iOS. Compatibility bugs lead to unpredictable user experiences, hinder adoption, and require thorough cross-platform, cross-browser, and multi-device testing to ensure consistent functionality.

Examples:

• Cross-browser issues: Web forms display or behave incorrectly in Safari or Edge while working in Chrome.
• OS version problems: Mobile apps crash on older Android versions but run smoothly on iOS.
• Screen size/layout issues: Overlapping text or broken layouts on different screen sizes or resolutions.
• Browser-specific functionality: JavaScript or CSS is not working in certain browser versions.

Perform compatibility testing across devices, browsers, and operating systems; verify that your website or mobile app works consistently for every environment your users might have.

4. Regression Bugs

Regression bugs occur when previously working functionality breaks after changes such as new features, updates, bug fixes, or code refactoring. Unlike new defects, regressions represent a backward step in software quality, often emerging at module integration points or due to overlooked dependencies.

They can affect critical workflows and user-facing features. Regression bugs emphasize the importance of re-running existing test suites, using version control, and verifying third-party updates to ensure that software continues to function as expected after modifications.

Examples:

• Search functionality failure: A previously working search stops returning correct results after adding a new module.
• Login regression: Login fails after updating the authentication module.
• Bug report generation: Previously accurate reports now display incorrect data.
• UI behavior regression: Buttons or menus stop functioning after adding a new feature.

Perform regression testing after every code change or release; retest previously working features to ensure new updates haven’t broken existing functionality.

5. Logical Bugs

Logical bugs occur when incorrect algorithms, flawed logic, or misunderstandings of business rules produce unexpected results in a software application, even though the code runs without syntax errors. Unlike functional bugs that break features outright, logical bugs allow features to operate while generating inaccurate outputs, making them particularly insidious.

These defects are dangerous because the application appears to function correctly on the surface, masking errors in calculations, decision-making processes, or workflows. In critical domains like finance, healthcare, or inventory management, logical bugs can cause significant business or operational consequences.

Examples:

• Conditional workflow errors: Users are routed to incorrect screens or given wrong permissions due to faulty logic in role-based access.
• Data processing mistakes: Reports, dashboards, or summaries showing inaccurate totals because of flawed aggregation or business rules.
• Assignment/routing errors: Support tickets, leads, or tasks assigned to the wrong users or queues due to incorrect conditional rules.
• Inventory discrepancies: Stock levels updated incorrectly in order or warehouse management systems, causing overselling or shortages.

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6. Syntax Bugs

Syntax bugs occur when code violates the rules of the programming language, such as missing semicolons, unmatched parentheses, incorrect variable names, or inconsistent indentation. These bugs prevent the software application from compiling or executing correctly, often causing immediate failures when the code runs.

Although they are the most basic type of programming error, syntax bugs can still appear in complex applications or under tight development timelines. Modern development environments and automated tools help catch these errors early, reducing their occurrence before the software reaches testing.

Examples:

• Missing punctuation or brackets: Forgetting a closing bracket } or semicolon ; in a function that prevents compilation.
• Incorrect variable names: Using userNmae instead of userName, causing undefined variable errors.
• Wrong function calls: Calling processData() with the wrong number of arguments or misspelling the function name.
• Inconsistent indentation: Test script failing due to misaligned blocks or tabs/spaces mismatch.
• Typographical errors in keywords: Using pritn() instead of print() in test scripts, causing immediate execution failure.

7. Performance Bugs

Performance bugs affect the speed, responsiveness, and scalability of a software application. They can degrade resource usage, slow down interfaces, or make the software application unreliable under high load, large datasets, or peak traffic conditions. These bugs usually do not stop the application from functioning, but negatively impact user experience and software stability.

Examples:

• Memory leaks: Mobile app becomes increasingly unresponsive over time because of unhandled memory usage.
• Slow data processing: Reports or dashboards take several minutes to load due to unoptimized algorithms.
• UI lag: Web page freezes when multiple users access it simultaneously.
• Search delays: Search functionality or filters respond slowly when handling large datasets.
• High resource usage: CPU or memory spikes during normal operations, affecting overall performance.

Use techniques like performance testing to measure response times, memory usage, and throughput; simulate real-world and peak loads to spot slowdowns before they affect users.

8. Security Bugs

Security bugs are vulnerabilities in a software application that can be exploited by attackers, putting sensitive data, system integrity, or user privacy at risk. These defects are among the most critical because they can lead to data breaches, financial losses, and damage to an organization’s reputation.

Examples:

• SQL injection: Inadequate input validation allows attackers to manipulate database queries.
• Cross-Site Scripting (XSS): Malicious scripts injected into web pages affect other users.
• Authentication bypass: Weak login or session management lets unauthorized users access accounts.
• Sensitive data exposure: Configuration files, credentials, or personal data visible or transmitted unencrypted.

Implement security testing, including penetration tests and vulnerability scans; actively try to break access controls and exploit inputs to uncover hidden vulnerabilities.

9. Heisenbugs

Heisenbugs are elusive defects in software applications that change behavior or vanish when someone tries to observe or debug them. Common in complex, concurrent, or multi-threaded systems, these bugs arise due to timing variations, memory allocation changes, or the act of debugging itself.

They are difficult to reproduce, making isolation and analysis challenging. Heisenbugs often occur in real-time systems, embedded software, or applications where execution timing is critical, requiring specialized, non-intrusive monitoring to detect without altering program behavior.

Examples:

• Log statement masking bug: Adding a debug log changes thread timing and hides the defect.
• Race condition: Variable updates yield inconsistent results depending on thread execution order.
• Memory allocation shift: Memory addresses change during debugging, hiding the problem.
• Timing-sensitive failure: Bug occurs only under specific thread-scheduling scenarios.
• Intermittent crash: Application crashes sporadically, making reproduction unpredictable.

10. Boundary Bugs

Boundary bugs occur when software fails at the edges of allowable input ranges, such as maximum/minimum values, array indices, or date limits. They typically arise from off-by-one errors, improper input validation, or unhandled edge cases.

They can cause crashes, incorrect outputs, or memory corruption, especially when processing large datasets, extreme values, or date transitions. Detecting them requires careful testing of edge conditions, including negative numbers, zero, maximum lengths, and overflow scenarios.

Examples:

• Array index out of bounds: Accessing an element beyond the array length causes crashes.
• Maximum file size failure: Uploading a file at the system limit triggers errors.
• Negative number handling: The system accepts negative inputs, where only positives are allowed.
• Date overflow: Calculations fail when processing month-end or year-end dates.
• Integer overflow: Arithmetic operations exceed the allowed range, causing incorrect results.

11. Concurrency Bugs

Concurrency bugs arise when multiple components of a software application execute simultaneously, causing unpredictable or incorrect behavior. They often appear in multi-threaded or distributed systems as race conditions, deadlocks, or resource contention issues.

They are difficult to reproduce because timing differences influence outcomes, and standard testing may not detect them. Detecting and fixing them requires specialized tools like race detectors, thread analyzers, and careful attention to synchronization, locking mechanisms, and coordination in distributed systems.

Examples:

• Deadlock: Two threads wait indefinitely for each other, halting execution.
• Race condition: Simultaneous access to a shared variable produces inconsistent results.
• Resource contention: Multiple threads compete for memory or CPU, degrading performance.
• Lost updates: Updates from one thread overwrite another’s changes.
• Thread-safe misuse: Improper use of thread-safe libraries causes an inconsistent application state.

Every bug follows a journey from discovery to closure. Learn more about the bug lifecycle.