One

One simply one. One crack one unguarded entry point of entry or one moment of opportunity. So true, and so deadly at the same time:

We only need to be lucky once. You need to be lucky every time.
— The IRA to Margaret Thatcher, after a failed assassination attempt.

Probably the most famous example of one point of weakness is Achilles heel (http://en.wikipedia.org/wiki/Achilles%27_heel). In short Achilles' mother dipped him in the river Styx, and the water from the River made him invulnerable. Except for where the mother held him with her finger and thumb on his heel. Thus he was invulnerable except one spot, his heel, which lead to his downfall.