chink in armor
chink in (one's)/the 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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.