Expressions (phrases, clauses, etc.), Noun
fly in the ointment; small flaw in otherwise perfect object; only trouble; only fault
Idiomatic expression
手八丁口八丁の彼だけど、誠がないのが玉に瑕だね。He works fast and is very articulate but his insincerity is his biggest defect.
Also written 玉に傷【たまにきず】