fly in the ointment
US /flaɪ ɪn ðɪ ˈɔɪnt.mənt/
UK /flaɪ ɪn ðɪ ˈɔɪnt.mənt/
1.
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.
•
His bad mood was the only fly in the ointment during the celebration.