First Deficiency Cited in Most Recent GAO Report (and it's being addressed). This number is wild. F-35 is 21% of Total DOD Acquisition Costs...

For example, the program is developing a new helmet mounted display, which will resolve an existing green glow effect that can distort a pilot’s vision during night time carrier landings. Program officials expect installation of some of the new displays in 2019. 

http://www.defense-aerospace.com/articles-view/release/3/202423/gao-reports-new-f_35-problems%2C-finds-costs-continue-to-grow.html