Chink in (one's)/the armor American British informal
To indicate a minor yet extremely damaging fault or weakness
Jane is bright but his hot temper has wrecked many business relationships. It's a chink in his armor.
Chemistry has always been a chink in my armor.
The senior's tendency to use the money to buy many new houses was the chink in the armor the police needed to accuse him of corruption.
A problem or difficult circumstance
I'm short-sighted, so forgetting my glasses is a real hair in the butter for my sight.