Fly in the ointment

In English, the phrase fly in the ointment is an idiomatic expression for a drawback, especially one that was not at first apparent, e.g.

The likely source is a phrase in the King James Bible:

For five centuries, 'a fly in the ointment' has meant a small defect that spoils something valuable or is a source of annoyance. The modern version thus suggests that something unpleasant may come or has come to light in a proposition or condition that is almost too pleasing; that there is something wrong hidden, unexpected somewhere.

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