A Machine With No Spare Parts
When the machine is running smoothly, the need for redundancy seems moot. Why have a backup system and pay for spare parts when what we have is working already? Why have two engines on a plane when one can keep it in the air?
It's only when the gear grinds to a halt, or the batteries run out, that we realize the importance of redundancy. Teams can run at capacity for a time, machines at full throttle temporarily, but there must also be a plan for relief.