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:[1]
For five centuries now '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.
This idiom has been used in the title of some books: The Fly in the Ointment: 70 Fascinating Commentaries on the Science of Everyday Life and The Fly in the Ointment by Alice Thomas Ellis.