throw a monkey wrench in the works

throw a (monkey) wrench in(to) the works

To disrupt, foil, or cause problems to a plan, activity, or project. We had everything in line for the party, but having the caterer cancel on us at the last minute really threw a wrench in the works! It'll really throw a monkey wrench into the works if the board decides not to increase our funding for this project.
See also: throw, work, wrench

throw a monkey wrench in the works

Fig. to cause problems for someone's plans. I don't want to throw a monkey wrench in the works, but have you checked your plans with a lawyer? When John suddenly refused to help us, he really threw a monkey wrench in the works.
See also: monkey, throw, work, wrench