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.
He is excellent, but his frequent lateness is the fly in the ointment.
This hotel has perfect service and rooms, but the distance from the station is the only drawback.
Standard kanji spelling. 瑕 means 'flaw' or 'blemish'.
Common variant using 傷 (wound, injury) instead of 瑕. Considered irregular kanji usage but widely seen.
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.