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.

We had a cookstove, beans, and plates; the fly in the ointment was the lack of a can opener.

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

Dead flies cause the ointment of the apothecary to send forth a stinking savour. (Ecclesiastes 10:1)

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.