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State of a software item that prevents it from performing as required (IEV 192‐04‐02).

Combined with a particular demand or trigger, the software bug may lead to item failure. Such a failure is a systematic failure and is sometimes called a software failure (see Figure 3.10). If the trigger is a random event, the software failure is random. Software bugs are difficult to reveal and software development projects therefore include a detailed process for finding and correcting bugs. This process is called debugging.

Software does not deteriorate and software bugs do not occur at random in the operational phase. They have been programmed into the software and remain until the software is modified. New software bugs are often introduced when new patches or new versions of the software are installed to remove known bugs. The same software failure occurs each time the same activation condition or trigger occurs. If relevant activating conditions or triggers do not occur, the software bug remains undetected. Installations of the same software may show very different frequencies of software failures because the failure frequency is proportional to the frequency of the occurrence of activating conditions or triggers.