chink in armor
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 really the chink in his armor.
chink in one's armor
Fig. a special weakness that provides a means for attacking or impressing someone otherwise invulnerable. (Alludes to an opening in a suit of armor that allows a weapon to penetrate.) Jane's insecurity is the chink in her armor. The boss seems mean, but the chink in his armor is that he is easily flattered.