throw a monkey wrench

throw a monkey wrench or throw a wrench  {v. phr.},  {informal}
To cause something that is going smoothly to stop.
The game was going smoothly until you threw a monkey wrench into the works by fussing about the rules.
The Michigan tacklers threw a wrench into the Wisconsin team's offense.
He hoped to see the class plan fail and looked for a chance to throw a wrench in the machinery.
Categories: informal verb