a fly in the ointment

The phrase grows out of Ecclesiastes 10:1: "Dead flies cause the ointment of the apothecary to send forth a stinking savour." A single dead fly, the Teacher observes, is enough to turn a whole jar of costly perfumed oil rancid.

The verse goes on to apply the image to reputation — a little folly can outweigh great wisdom and honour, just as a small corruption ruins something valuable.

The exact phrase "a fly in the ointment" is a later English coinage distilled from the verse, and it kept the underlying idea intact: one small defect can spoil an otherwise excellent thing. A fly in the ointment is the minor snag that mars a good plan, a nice day, or a promising arrangement.