a fly in the ointment a minor irritation or other factor that spoils the success or enjoyment of something.

☞ This expression alludes to Ecclesiastes 10:1: 'Dead flies cause the ointment of the apothecary to send forth a stinking savour'.

2013New Zealand Herald The X2 wins on a design basis, and its battery life led the pack hands down thanks to the additional battery in its keyboard dock. About the only fly in the ointment is the lack of any real processing muscle.