fly in the ointment idiom informal
美中不足之处；令人扫兴的东西（或人）
a single thing or person that is spoiling a situation that could have been very positive or enjoyable
I'm looking forward to Sunday, the only fly in the ointment being the fact that I'll have to sit next to my mother-in-law. 我期待着星期日的到来，唯一令人不快的是，到时我必须坐在丈母娘旁边。