Death star syndrome
When something massive and extremely powerful has one tiny weakness that, if you exploit, the entire thing will go to sh*t
The Alien Invaders from "Independence Day" suffer from Death Star syndrome: if the mothership's cockpit is destroyed, all of the other ships will blow up.
Disclaimer: this example's weakness may not be that tiny, but you get the point
Disclaimer: this example's weakness may not be that tiny, but you get the point