Fly in the ointment

A small but irritating problem that threatens to spoil everything, the source is the Bible Ecclesiastes 10:1, “Dead flies cause the ointment of the apothecary to send forth a stinking savour: so doth a little folly him that is in reputation for wisdom and honour.” In ancient times, flies were attracted to apothecaries’ ointments, would become stuck and die, thus spoiling the ointment. The exact phrase fly in the ointment and its current figurative meaning dates from the early 18th century.