an irritating factor

fly in the ointment = ein Haar in der Suppe

ointment = Salbe, Balsam

EXAMPLE SENTENCE : A study found that companies in general are increasingly leveraging the cloud to deliver business-critical applications. However, there’s A FLY IN THE OINTMENT. Most companies aren’t doing it right. (ABA Banking Journal) --- For some housing-market analysts there's a big FLY IN THE OINTMENT that could spoil the housing market recovery for us all: rising interest rates. (Time magazine)