I stole borrowed that quote from an Aviation Week & Space Technology article discussing the software problems facing the F-35, Jpint Strike Fighter (One aircraft for Air Force, Navy, and Marine use [all with different needs, of course]). Airborne, ground, simulator, and mission planning data must be running in one huge application in many different environments and interactions with other airborne devices may require almost instantaneous updates.
The code must be checked / validated / tested every time a change is made. One commenter stated:"... The best debugged code in history is reputed to be the Space Shuttle code which is about 410,000 [Single Lines of Code]. It is believed that there is 1 UNDISCOVERED bug in every 10,000 lines." No one knows, right now, how many lines of code are in the JSF, but an over-site organization claims the lack of operational software is delaying the actual production of the aircraft.
Writing the software is the easy part, testing it becomes exponentially harder as the size/complexity increases. When the software is critical to the life of a human, there is no such thing as an "little bug"!