If someone or something throws a wrench into the works or throws a monkey wrench into the works, they cause problems which prevent something from happening in the way that was planned. Of course they may not sign the agreement by the sixteenth and that would throw a monkey wrench into the works. Note: Instead of saying the works, people often describe the situation in which the problem is caused. Most health-related problems, of course, are not life-threatening, but they can throw a wrench into an otherwise pleasant holiday. The US delegation threw a giant monkey wrench into the process this week by raising all sorts of petty objections. Note: The usual British expression is throw a spanner in the works.