expression, noun
fly in the ointment; small flaw in an otherwise perfect thing; only fault
Idiomatic expression used when something excellent has one minor defect. Often used in the pattern ～が玉に瑕 or ～は玉に瑕.
彼は優秀だが、遅刻が多いのが玉に瑕だ。
He is excellent, but his frequent lateness is the fly in the ointment.
このホテルはサービスも部屋も完璧だが、
This hotel has perfect service and rooms, but the distance from the station is the only drawback.
傷 is a general word for wound, injury, or flaw. 玉に瑕 specifically refers to a minor flaw in an otherwise excellent thing, using the metaphor of a blemish on a jewel.
瑕 is a less common kanji for flaw or blemish, mainly used in this idiom. It carries a more literary or classical nuance compared to 傷.
Literally 'a flaw on a jewel'. The idiom comes from the idea that even a beautiful gem can have a tiny imperfection, used metaphorically to describe a small defect in an otherwise excellent person or thing.