Similar to "chink in one's armor. A "chink" means "a narrow opening or crack", and a chink in one's armor means a weak point that makes you vulnerable to attack.
But "chink" is also an offensive term for a Chinese person, so "chink in one's armor" can be misunderstood, especially if what you are talking about actually involves a Chinese person. ESPN got in trouble a couple of times with it, even though they had used it thousands of times before without incident, because of a couple articles where the weakness they were describing on some team was a player who was Chinese.