Idioms, Fallacies, and Paradoxes

Achilles' Heel/Chink in One's Armour

Achilles' heel:

A fatal weakness in spite of overall strength, which can lead to downfall. While the mythological origin refers to a physical vulnerability, idiomatic references to other attributes or qualities that can lead to downfall are common.

Chink in one's armour:

Refers to an area of vulnerability. It had traditionally been used to refer to a weak spot in a figurative suit of armour.