fly in the ointment

fly是苍蝇，ointment是药膏。本来好好的药膏上面趴了一只苍蝇，就用来比喻本来很好很完美的东西却因为一个小问题而不那么完美了。

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a single thing or person that is spoiling a situation that could have been very positive or enjoyable

美中不足之处;令人扫兴的东西(或人)

The new computers are much better than our old ones but the fly in the ointment is that they are prone to overheating.

新电脑比我们的旧电脑好得多，但美中不足的是它们容易过热。