fly in the ointment 小麻烦
Any small defect that ruins something valuable; an annoyance.
We have an efficient, modern office, but the fact that the paper in our computer printer jams frequently is the fly in the ointment that often slows down our work.
我们拥有高效的现代化办公室,但有个小麻烦:我们的电脑打印机总是卡纸,经常妨碍我们工作。