The complexity of maintenance error can range from simple errors, like a single aircraft maintenance technician forgetting to torque a finger-tightened screw, to errors causing system-wide failure. In the case of a significant maintenance system breakdown, the primary maintenance task was incorrectly performed, followed by other erroneous procedures, causing the error-tolerant maintenance system to break down significantly. To better understand the various types of error, researchers have closely examined human error and proposed various models and theories.