a fly in the ointment
the only thing that spoils something and prevents it from being successful

[TahlilGaran] Dictionary of Contemporary English ▲

fly in the ointment
someone or something that spoils a good situation.
What remains a fly in the ointment is the fact that the UN has not approved the peace plan.

Etymology: from a Bible story which explains that dead flies spoil ointment (= medicine spread on the skin)

[TahlilGaran] English Idioms Dictionary ▲

fly in the ointment
a small thing that spoils one's enjoyment of something; something small that spoils one's fun
The problem with the music was a fly in the ointment during the wedding ceremony.

The complaints about the food were a fly in the ointment and gave everyone a bad feeling about the gathering.

[TahlilGaran] English Idioms Dictionary ▲

fly in the ointment
n. phr., informal An unpleasant part of a pleasant thing; something small that spoils your fun.
We had a lot of fun at the beach; the only fly in the ointment was George's cutting his foot on a piece of glass.
Your new job sounds too good to be true
■ interesting work, high pay, short hours. Isn't there any fly in the ointment?

[TahlilGaran] English Idioms Dictionary ▲