There are many ways in which a component or structure may fail. It can be either physical fracture, or functional failure. Either type of the failures can be caused by fatigue, corrosion wear environmental exposure or other mechanisms. Everything is expected to fail over time, but if a failure occurs prematurely, then the major question to be answered is what caused the failure: human error, improper workmanship, negligence, or customer abuse.
The most typical causes of premature failure are:
Material/component related issues
Manufacturing process related issues