TANKS
Yeah, I gotta get that cut and paste thingie down, but anyway, look at what happened to Apollo 13 and thru sheer luck and cricumstance, we saved the crew and valuable lessons were learned. The Oxegen tank from 13 was swapped off apollo 10 and one could just imagine the consequences of a 13 type of accident happening prior to moon landing.
So weakest links aside, it also can be the human error factor that can contribute to mission failure even though paperwork was signed off on.