Большой русско-английский фразеологический словарь
Л128
ЛОЖКА ДЁГТЮ В БОЧКЕ МЁДА (у) (saying)sth.that spoils an otherwise good, satisfactory state of affairs (said in vexation when some insignificant but unpleasant thing spoils what is otherwise very good-a project, mood, impressionetc): - a fly in the ointment.