single point of failure

noun2

1. •
   A component in a device, or a point in a network, that, if it were to fail, would cause the entire device or network to fail (normally eliminated by adding redundancy).

   • The Jesus nut got its name because it is a single point of failure — if it goes, there's nothing you can do but pray.
2. •
   A minor mistake that creates a chain reaction.

   • The intact twine in Dylan's beef Wellington was a single point of failure.