a chink in somebody's armour
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.
a chink in somebody’s ˈarmour(British English) (American English a chink in somebody’s ˈarmor) a weakness in somebody’s argument, character, etc., that can be used in an attack: The one chink in her armour is the lack of a sense of humour. She hates people laughing at her.
A chink is a small hole.