fly in the ointment
US /flaɪ ɪn ðɪ ˈɔɪnt.mənt/
UK /flaɪ ɪn ðɪ ˈɔɪnt.mənt/

1.
美中不足, 瑕疵
a minor irritation or flaw that spoils an otherwise pleasant situation
:
•
The trip was perfect, except for one fly in the ointment: the weather was terrible.
這次旅行很完美,除了一個美中不足:天氣很糟糕。
•
His bad mood was the only fly in the ointment during the celebration.
他的壞脾氣是慶祝活動中唯一的美中不足。