FLY IN THE OINTMENT

{n. phr.}, {informal} An unpleasant part of a pleasant thing; something small that spoils your fun. * /We had a lot of fun at the beach; the only fly in the ointment was George's cutting his foot on a piece of glass./ * /Your new job sounds too good to be true - interesting work, high pay, short hours. Isn't there any fly in the ointment?/