Back in my Boscombe Down days, we were pushing to have all software on which the safety of the aircraft depended subjected to static code analysis (proof of software correctness). We had some success with this on the C130J project, but were told that the upgraded Harrier HUD didn't need to be developed in this manner as the HUD wasn't the "primary source of speed, attitude, and altitude information" for the pilot. Instead, he was supposed to refer to the basic air/gyro operated instruments, which were mostly obscured by his knees and certainly not directly in his line of sight.
In those days, Their Airships used to make it up as they went along, and it seems times haven't changed much!