So, we can say a fly in the ointment to point to something bad that ruins something else.

所以，我们可以说a fly in the ointment来指虽小却搅了全局的东西或事情。

But there is a fly in the ointment.

但美中不足。

At the risk of being a fly in the ointment...

还得小心那颗老鼠屎。