In short: Bug, defect, anomaly, failure, incident, error—the generic term “bug” encompasses a whole vocabulary that you need to master to test effectively. Each term refers to a specific reality, ranging from the cause in the code to the effect visible to the user. This guide clarifies these nuances, explores the origin of the word “bug,” and helps you speak the same language as your QA teams.
There’s nothing quite like a 404 error or a button that doesn’t work to frustrate a user, right? Yet behind the generic term “bug” lies an entire technical lexicon that’s essential for anyone who wants to successfully navigate the world of automated testing and QA.
If you're a little confused about the differences between a bug, an error, a defect, and a failure, this guide is for you. Let's explore the nuances ofcomputer anomalies together.
Fun Fact: Where does the word “bug” come from?
Do you know where the word “bug” comes from? It’s no coincidence that this English word—which means “insect” in French—was chosen to describe a computer problem. Inventor Thomas Edison was the first to use this term. According to computer scientist Isabelle Collet (The Forgotten Women of Digital Technology), Thomas Edison reportedly discovered a malfunction in his phonograph, which he likened to an imaginary insect hidden inside the device, responsible for the problem.
Sixty years later, while scientists were working on the Mark II—the electromechanical computer that succeeded the Mark I (IBM’s first digital computer, built in the United States)—an incident occurred. Grace Hopper, an engineer on the team, recorded the story in the computer’s logbook. “On September 9, 1947, you can see, stuck to the page, the carcass of an insect that had flown into a switch and gotten stuck,” she explains. On the page, beneath the insect, a small handwritten note reads “the first recorded bug.”
But what exactly is a bug?
Originally, the term “bug” referred to a technical malfunction—a computer glitch caused by a short circuit, for example. But since then, its meaning has changed significantly.
Today, the term “bug” mainly refers to a malfunction. In other words, we can say there is a bug whenever the behavior of a program or software does not match the expected result. It may be caused by a computer-related issue, such as an error in the program’s code.
For example, in the context of an app or a website, we refer to a bug when a button doesn’t appear or when nothing happens when you click on it, when the app crashes suddenly, when the text color is wrong, when the product price displayed doesn’t match the price in the shopping cart, or when the shopping cart empties when you log in to your account.
And even that is just a small sample of the bugs you might encounter.
Bug, defect, anomaly, failure—are they all the same thing?
But what is the difference between a bug, an anomaly, a defect, and a failure? In reality, one could say that the term “bug” is a generic term that encompasses several levels, depending on the resulting impact.
>> A defect: This is generally a simple deviation from the specifications determined earlier. It can be a graphical defect, such as when the displayed color is incorrect or when there is a misalignment in the visual elements. But it can also be a text-related defect, such as when a word is misspelled or does not match the one chosen to label a particular button. Note that a defect does not necessarily lead to a bug or a failure, and this article from La taverne du testeur explains this very well!
>> A defect: This refers to a defect that negatively and significantly disrupts the software’s operation and may therefore affect its use. The software may not behave as it is supposed to according to the requirements of the specifications, design documents, etc., which can be problematic. A button that doesn’t work or redirects to the wrong page, a shopping cart that empties itself, or an inability to log in to an account can all be classified as anomalies.
>> A failure: This is the most severe type of bug. It means that part or all of the software is unusable, either because it does not work or because its behavior differs too significantly from what is expected, thereby rendering it unusable.
And what about incidents? Mistakes?
In the world of testing, you may also sometimes hear the terms “incident” or “error.” You might be wondering: How are they different from bugs?
It’s simple. An incident is a problem that was detected while running a test. In other words, the test failed because it identified a deviation from the expected behavior (a defect), but we don’t yet know why. It could be an anomaly or a failure, such as a false positive—that is, when a defect is reported even though it does not actually exist in the tested component. You must then manually reproduce the test sequence to verify what caused the incident and determine whether it is a genuine bug.
As for errors, one could say that they are what give rise to defects. In the sense that they are human actions carried out incorrectly—such as a mistake in the design of an application or in the writing of code—that cause the software to ultimately fail to produce the expected result. All that is needed, then, is to correct the error to fix the defect, or to modify the specifications so that the defect is ultimately incorporated as an enhancement.
In reality, regardless of the severity or significance of bugs, the key principle to keep in mind is that they must be identified and fixed as quickly as possible, as they can have a direct impact on your business—whether in terms of reputation, finances, or legal issues. For example, 86% of French people abandon a purchase on an e-commerce site because of a bug, according to our 2020 survey.
That’s why it’s so important to thoroughly test your app or website before launching it and to continue doing so regularly using automated functional testing!
FAQ
What is the difference between a bug, a defect, and an anomaly?
A defect is an error in the code; an anomaly is a discrepancy between expected and actual behavior; and a bug is the common, colloquial term for these malfunctions. Distinguishing between them helps with accurate diagnosis and communication.
What is the difference between a bug and an incident?
A bug is a software defect; an incident is the service interruption or degradation that it causes in production. A single bug can cause multiple incidents, and not all incidents are caused by a bug.
Where does the word “bug” come from?
The term gained popularity after the discovery in 1947 of a real insect stuck in a computer relay, as recorded in a logbook. Since then, “bug” has come to refer to any system malfunction.