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An error message is information displayed when an unexpected condition occurs, usually on a computer or other device. On modern operating systems with graphical user interfaces, error messages are often displayed using dialog boxes. Error messages are used when user intervention is required, to indicate that a desired operation has failed, or to relay important warnings (such as warning a computer user that they are almost out of hard disk space). Error messages are seen widely throughout computing, and are part of every operating system or computer hardware device. Proper design of error messages is an important topic in usability and other fields of human–computer interaction.
The following error messages are commonly seen by modern computer users:
With the rise of Web 2.0 services such as Twitter, end-user facing error messages such as HTTP 404 and HTTP 503 started to be displayed with whimsical characters, termed Fail Pets or Error Mascots. The term "Fail Pet" was coined, or at least first used in print, by Mozilla Engineer Fred Wenzel in a post on his blog entitled "Why wikipedia might need a fail-pet — and why mozilla does not." Dr. Sean Rintel argues that error messages are a critical strategic moment in brand awareness and loyalty. Fail pets are of interest to marketers because they can result in brand recognition (especially through earned media). "However, that same recognition carries the danger of highlighting service failure." The most famous fail pet is Twitter's Fail Whale (see Twitter service outages). Other fail pets include:
The form that error messages take varies between operating systems and programs.
Error messages on hardware devices, like computer peripherals, may take the form of dedicated lights indicating an error condition, a brief code that needs to be interpreted using a look-up sheet or a manual, or via a more detailed message on a display.
On computers, error messages may take the form of text printed to a console, or they may be presented as part of a graphical user interface. Error messages are often presented as a dialog box, which makes them to cause a following mode error in the user interaction. In many cases the original error can be avoided by error prevention techniques. Instead of raising an error message the system design should have avoided the conditions that caused the error.
While various graphical user interfaces have different conventions for displaying error messages, several techniques have become common:
The three main factors that influence the design of error messages are technical limitations, the amount of information to be presented, and what kind of user input is required.
Some systems have technical limitations that may constrain the amount of information an error message can contain. For example, a printer with a sixteen-character alphanumeric display can only show a very limited amount of information at once, so it may need to display very terse error messages. Even with computer monitors, the programmer must consider the smallest monitor that a user might reasonably use, and ensure that any error messages will fit on that screen.
The nature of the error determines the amount of information required to effectively convey the error message. A complex issue may require a more detailed error message in order to adequately inform the user of the problem.
When designing error messages, software designers should take care to avoid creating security vulnerabilities. The designer should give the user enough information to make an intelligent decision, but not so much information that the user is overwhelmed or confused. Extraneous information may be hidden by default or placed in a separate location. Error message should not expose information that can be exploited by a cracker to obtain information that is otherwise difficult to obtain. Examples are systems which may show either "invalid user" or "invalid password" depending on which is incorrect, and the error page in the web server IIS 5.0 which provides a complete technical description of the error including a source code fragment.