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.
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Source: A Dictionary of American Idioms
