Human error in maintenance has two basic forms. In the first case, error results in a specific aircraft discrepancy not existing before the maintenance task was initiated. Examples include incorrect installation of line-replaceable units, failure to remove a protective cap from a hydraulic line before reassembly, or damaging an air duct used as a foothold to perform a task. These examples also illustrate mismatches in the L-H interface of the SHELL model. The second type of error results in an undetected unsafe condition while performing a scheduled or unscheduled maintenance task to detect aircraft degradation. Examples include a structural crack unnoticed during a visual inspection task, a faulty avionics box remaining on the aircraft because incorrect problem diagnosis led to removal of the wrong box. These errors may have been caused by poor ergonomic tool design, which would indicate an L-H mismatch or by incomplete documentation, an L-S interface flaw.