chink in one's armor

A minor but very detrimental flaw or weakness. Yeah, he's brilliant, but his violent temper has destroyed many business relationships—it's a real chink in his armor. The criminal's tendency to use his own cell phone to conduct business was the chink in the armor the police needed to put him in jail.

A vulnerable area, as in Putting things off to the last minute is the chink in Pat's armor and is bound to get her in trouble one day . This term relies on chink in the sense of "a crack or gap," a meaning dating from about 1400 and used figuratively since the mid-1600s.

A vulnerable spot, a weakness. The term alludes to the medieval knight’s armor made of mail—interlinked rings of metal jointed at various points. When a crack, or chink, developed between the links or joints, he was less protected against a spear or arrow. The noun “chink” has been used figuratively for such a fissure since the 1600s, and the current term came soon afterward. See also Achilles' heel.