An additional caveat is that if this is true, the reverse is also true. That is, lack of one nail could also have good consequences, or no concequences:
For want of a nail, the shoe was lost;
Whilst one nail was missing, this was not a problem since there were four others holding the shoe on.
For want of the shoe, the horse was lost;
Lacking horseshoes, the rider decided to ride on the grass beside the road, not on the road. Consequently, he avoided the hail of arrows launched at that section of the road by the enemy.
For want of the horse, the rider was lost;
Since he had no horse, Joe the Messenger stayed at the camp, and hence wasn't killed by the ambush the enemy had set up for just this situation.
For want of the rider, the battle was lost;
Lacking a horse, Joe the Messenger remained at the camp, and took part in the battle. His messenger's running skills allowed him to circumnavigate the enemy ranks and kill the enemy commanders, causing the entire enemy army to flee in disarray, believing they were being attacked from all directions, and were hopelessly outnumbered.
For want of the battle, the kingdom was lost;
Having lost it's best troops in battle, the kingdom was not able to invade the nearby country to rape, pillage and slaughter people of other religions.
At every point of interaction, there is a possiblility of mistakes or failure. But as levels of interaction accumulate, responsibility is shared out. Another example:
In a war, a cook takes special care of his knives, making sure he has one set (blue handles) for cutting uncooked meat and another set (red handles) for cutting cooked meat. This prevents bacteria (That could have been killed by cooking) infecting soldiers. This keeps them healthy and alert, and able to partake in the 'big push' that wins the war, instead of sitting around being sick and getting killed. Sure, the cook may well be helping to win the war by keeping his knives clean, but other things more directly lead to victory, i.e. having good tactics, competent generals, not stubbornly sending wave after wave of soldiers to get mowed down by enemy machine guns, etc.
If the 'big push' fails, it is much more likely to be the fault of generals than cooks.
Anyway, here's my point: If a plan fails on a single point, it is as much the fault of the person who made up the plan as it is the person who failed. As long as people do their jobs properly, they cannot be blamed for 'unforseeable results'. "We'd just be following a standing order; no court-martial there."