A chain is only as strong as its weakest link. This is a somewhat overused expression, but goes unappreciated by some of the people modelling fatigue. It is possible to be very careful about all aspects of the structural design, determine the material characteristics precisely, yet not appreciate the degree to which the actual structure is susceptible to error during construction.
An example of an error which resulted in catastrophic failure was the case involving a light aircraft [1] in 1996. A weld was improperly completed on the nose wheel, which led it to sheer off on contact with the runway. The failure was due to fatigue cracking which had been initiated when the weld was carried out.
For this reason, it is most important that the overall picture is concerned with the very real part that the manufacturing process plays in the determination of system reliability.