Human beings make errors (mistakes), which produce defects (faults, bugs), which in turn may result in failures. Humans make errors for various reasons, such as time pressure, complexity of work products, processes, infrastructure or interactions, or simply because they are tired or lack adequate training.
Defects can be found in documentation, such as a requirements specification or a test script, in source code, or in a supporting artifact such as a build file. Defects in artifacts produced earlier in the SDLC, if undetected, often lead to defective artifacts later in the lifecycle. If a defect in code is executed, the system may fail to do what it should do, or do something it shouldn’t, causing a failure. Some defects will always result in a failure if executed, while others will only result in a failure in specific circumstances, and some may never result in a failure.
Errors and defects are not the only cause of failures. Failures can also be caused by environmental conditions, such as when radiation or electromagnetic field cause defects in firmware.
A root cause is a fundamental reason for the occurrence of a problem (e.g., a situation that leads to an error). Root causes are identified through root cause analysis, which is typically performed when a failure occurs or a defect is identified. It is believed that further similar failures or defects can be prevented or their frequency reduced by addressing the root cause, such as by removing it.