Edge case

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An edge case is a problem or situation that occurs only at an extreme (maximum or minimum) operating parameter. For example, a stereo speaker might noticeably distort audio when played at maximum volume, even in the absence of any other extreme setting or condition.

An edge case can be expected or unexpected. In engineering, the process of planning for and gracefully addressing edge cases can be a significant task, and yet this task may be overlooked or underestimated.

Some common causes of edge cases^[1] are:

• Unpredictable user behavior
• Evolution of use cases (e.g. user behavior may change over time)
• Limited test coverage
• Product complexity (for instance, in distributed systems or microservice architectures)
• Resource limitations (e.g. limited processing power, computer memory, or computer storage)
• Other external causes

Some basic examples of edge cases include:

• A long username in an app overflows and displays incorrectly
• A booking system does not handle reservations correctly on a leap day (February 29th)

Non-trivial edge cases can result in the failure of an object that is being engineered. They may not have been foreseen during the design phase, and they may not have been thought possible during normal use of the object. For this reason, attempts to formalize good engineering standards often include information about edge cases.