I'd guess stuff like the F-35s STOVL system (http://en.wikipedia.org/wiki/Rolls-Royce_LiftSystem) to be a quite complex thing (but apparently the gun of the plane is more complex than this, lol) or the Airbus A380 and bigger planes due to the massive weight of the planes and the corresponding immense power of the engines.

The F35 has to spend something like 10 hours in the shop for every hour of flight time.

What the ...? Okay, at least that explains why according to wikipedia they plan to buy 1.7k jets... if they need 9 jets as "stand in" for every one that's been flying.