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:^[1]

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.

Just having the meaning of a phrase does not always explain away the fly, they are pesky rascals. Sometime hard to get away from for very long.

Try and find ways to keep the flies out of your ointment on your journey†nada te turbe†jim