Examples of unspecified wear and tear:

• Robotic handling in large warehouses is prone to a number of drops and/or vibration; quick acceleration of the product being handled may go beyond the anticipated design criteria, especially for internally mounted subsystems.
• A handheld product has a user accessible battery compartment - the battery cover must be in place for proper product operation, but because the cover is small and not secured, it was easily lost.
• A shipping package was not robust enough to protect a product during transportation within a backcountry environment that was never anticipated by the developer.
• An electromechanical device was designed (and vigorously tested) for use in 'expected' environments, but the product was taken into a very rugged environment, "because it was portable". It fell out of calibration after one week and, because it was not designed to be calibrated in the field, it became useless.
• A product was opened to repair an internal part and left open over a weekend. The product incurred considerable damage because the cleaning crew 'aired out' the facility during a dust storm that weekend.
• A product with electronic subsystems and moving mechanical components had an open housing. It resided in a damp environment under large temperature changes and experienced unexpected condensation and expansion/contraction that ultimately rendering the product inoperative.

You get the picture... wear and tear that is unanticipated can undermine a product's usefulness.