Wiring harnesses for the B-17 World War 2 bomber (Click for full-size)
In over 25 years of electronics troubleshooting on a wide variety of analog and digital equipment, some of it cutting edge, I have concluded that a majority of failures were due to failed connections. That is, components such as switches, plugs and jacks, crimped wire terminals, circuit board conductor fabrication, and solder joints failed more often than the components they connect. In the 21st Century (I like saying that) connectors, other than optical systems, still rely on old technology: mechanical contact between conductors.
The good news is that Non-mating Connectors (NMCs) use a totally different technology to transfer signals and power, and hold great promise for increasing the reliability of electronic equipment. Read more..(in Reliability section)