Since the program's earliest days the F-35, as with any new aircraft production program, has been plagued with parts shortages, defective parts and components, rework required and in many cases parts that required re-design. Planes are assembled with missing parts or with parts that have to go back and be replaced.
Suppliers struggling to meet deadlines are forced to cope with re-designed parts or to solve technical and reliability problems at their level. All of these factors, many of which stem from overly optimistic assumptions years ago, have added to the delays and rising costs of the F-35 program.
Ripping off a Dennis Miller Twitter post from earlier in the day and converting it: I sometimes feel that Robert Stevens has been sent from the heavens with his robo-aide Gort to rid the planet of the concept of caveat emptor.