Software Testing Essentials: Bug Types & Other Defect Characteristics

Anna Protopopova

Anna Protopopova

IT copywriter


30 Jul 2021

Reading time: 15 minutes

30 Jul 2021

If you’re involved in a software development process, you’ll inevitably have to deal with bugs. Code is rarely written perfectly from the beginning. That means that you will likely need to identify, record and resolve bugs. If bugs are not resolved in the early stages, they will disrupt the workflow later, and it will be far more time-consuming to fix them. As code gets bigger, looking back on old code to fix bugs will be more challenging.

However, if you’re aware of the most common types of bugs or defects you’re likely to encounter, you can tackle them earlier, faster, and more effectively.

In this article, based on our 19 year of software testing experience, we’ll discuss the most common types of software bugs or defects encountered in software engineering so that developers and testers can deal with them better.

What is a software defect?

Let’s move on to the software bug definition. It’s a common term in the computing environment. It’s used to describe when software fails. Some are easy to find, but others are difficult to identify because they’re contained in sections of the written code that operate under many conditions that rarely coincide in the course of running a program. This usually occurs due to unpredictable user actions, incorrect data entry, or other illogical actions. Software testing specialists collect information about problems in special programs and help in finding and fixing bugs. They offer effective solutions by providing high-quality computer systems.

What is a software defect

Common types of bugs

There are many different types of defects. Some relate to the security features of the system, others affect the performance of the product. One way or another, they all fall into one of the categories that we describe below. This bug classification allows you to more effectively find and eradicate them.

Algorithmic defects

These are defects that are associated with the sequence of functions that the program performs. If the logic is broken, an algorithmic or command error happens. These include different specific cases: wrong algorithm, missing command, incorrect data type, or programming errors. Therefore, to correct these defects, it’s necessary to restore the correct order of actions.

Functional defects

If the behavior of computer programs doesn’t correspond to certain functionality, it’s called a functional defect. For example, some button doesn’t perform a prescribed action. These defects are easily found during functional tests.

Non-functional defects

Non-functional testing is the testing of properties not related to the system’s functionality. These properties are determined by non-functional requirements that help find reliability, scalability, security, portability, or compatibility defects. This group includes the following examples of bugs:

  • Performance defects. Problems related to the stability of work, speed, response time, and resource consumption. The user may think that something is wrong with the operating system, but sometimes the problem’s actually in the software itself.
  • Security defects. This includes weaknesses that could allow attackers to access data. For this type of defect, there’s also a special type of test that allows you to identify possible errors and eliminate them.
  • Usability defects. By now, all good designers and developers realize the importance of usability for their work. Usable websites offer a great user experience, which leads to happy customers. Delight and satisfy your visitors with smart UX decisions, instead of frustrating and annoying them with design defects.

If it’s difficult for a user to sign up for a program or view content, then you’re dealing with a usability defect. The user should be comfortable using the software, so even if there are no bugs in other areas, the usability aspects should also be considered.

Hidden software bug types

Unfortunately, some bugs are not as obvious as the ones described above. It will take more time and effort to implement proper bug tracking. In addition, developer and tester experience play an important role in dealing with these defects. Let’s take a closer look at them.

Latent defects

A latent defect is a flaw in software that’s not defined until a user performs a set of operations. This bug is only revealed when a specific task has to be performed without normal scenarios or under unusual circumstances. This defect is usually detected during the product’s operation, so it’s important to thoroughly test all possible scenarios to avoid such complications.

Masked defects

Some bugs are hidden under other software defects. In other words, they can’t be detected until some other error that “hides” this disguised defect occurs. You’ll have to fix the primary problem to find the bug hidden behind it.

Cascading defects

In some cases, particular defects can cause problems in other areas. If the first bug is not found during testing, it can cause a chain reaction and create several more. In this case, the tester must be experienced enough to not only detect each bug but also to find their root cause.

Defect severity & priority

Ideally, your task would be to consistently catch each bug and get rid of them quickly. In reality, this is not always possible, and the stages of work have to be prioritized. As a result of this approach, those defects that cause the most damage to the software (possibly preventing the program from starting at all) have the highest priority. Those bugs that affect smaller parts of the software are of lower priority and can be fixed later.

Priority of bugs can be categorized into three parts:

Types of priority in software testing


However, it’s important to consider not only the priority but also the severity of the defects. The more seriously they affect the operation of the program, the greater the level of severity they have. 

In software testing, the types of severity of bugs can be categorized into four parts:

Types of severity in software testing

Distinguish priority from severity: priority is determined by the needs of the user, while severity depends on the functional characteristics of the product.

Defect probability

Some users may be lucky, and they will not face a particular problem. Defect probability is also called defect visibility, bug probability, or bug visibility. It can be different:

  • High. Most users are faced with a defect.
  • Medium. About half of those who interact with the functionality are faced with a defect.
  • Low. Almost no one encounters a bug.

The probability of a defect doesn’t apply to the entire program, but to a specific feature. This allows you to purposefully work on individual parts of the code and prioritize errors.

How to calculate defect density

The defect density is the ratio of the number of bugs found in software over a certain period to the size of the program or its individual part. For example, some developers calculate the density of bugs per 1000 lines of code. Density depends on the complexity of the code, the type of defects taken into account, and the skills of the developer and tester.

Let’s say a program consists of three modules. You can calculate the density of bugs in it. For example, bugs were distributed among modules as follows:

  • Module 1: 10 bugs,
  • Module 2: 20 bugs,
  • Module 3: 10 bugs.

In total, the program contains 40 bugs.

Now you need to consider the size of the modules:

  • Module 1: 1000 lines,
  • Module 2: 1500 lines,
  • Module 3: 500 lines.

In total, the program consists of 3000 lines.

To calculate the density of defects, you need to divide the number of bugs by the number of lines:

  • 40/3000 = 0.013 defects per line.

By calculating the density of defects, you can improve development processes, determine the quality of the software, and roughly understand how much work remains to be done.

Defect management process in software testing

Defect management is a systematic, regular process of finding and fixing bugs. It usually consists of the following stages:

  • Discovery of defects. At this stage, testing teams determine the presence of a defect. They need to find as many bugs as possible to spend the resources remaining after fixing them on small and quick improvements.
  • Defect categorization. It’s important not only to find, but also to refer the found bugs to a certain group. This will help you work with them more systematically and quickly.
  • Fixing defects by developers. Once the work pool is defined, developers can fix any bugs that fall within their purview. They make bug reports on the work done and, together, they eliminate all the identified defects.
  • Verification by testers. After working on bugs, you need to check if everything’s working correctly. At this stage, testers join the work again to track unresolved and new defects.
  • Defect closure. If the defect is successfully fixed, the testers mark it as closed. If not, then this part of the program is sent for revision so that the developer can eventually get rid of the problem.
  • Reporting. At this stage, testers collect the information about working with bugs in one place and report it to all team members. This allows them to keep the data up-to-date and track the work on the project.

Defect management process in software testing

What is software testing?

Now that you’re familiar with common software bug types, let’s make it clear why it’s so important to detect and eliminate them as well as perform software testing. Basically, the software testing process is aimed at verifying that an actual software product meets expected requirements and is free from defects. Before a new version of a computer program, website or mobile application gets to the user, it must go through the hands of test engineers. They perform automated and manual testing to look for places in the code where the program does not work as intended. To find as many bugs as possible, testers simulate different situations that may arise when using the application.

Software testing should start early in the Software Development Life Cycle. This helps to capture and eliminate defects in the early stages of SDLC i.e requirement gathering and design phases. An early start to testing helps to reduce the number of defects and ultimately the rework cost in the end.

According to HackerRank, the most common error found by testers and different developers is the distribution of untested or incorrect code, accounting for 60% of all bugs. Testing helps avoid these kinds of problems and makes the life of the product owner easier.

Trust Azoft in software testing

Over 19 years of work, we’ve created more than 800 projects, and worked meticulously on their testing and improvement. We work with products in different areas: insurance, logistics, media, retail, finance, among others. During the work process, we use an agile software development methodology, so you will be able to control the project at every stage, including testing. Email us at info@azoft.com to start working on creating high-quality software.