Monkey wrench

2. something that interferes with functioning; obstacle: He threw a monkey wrench into our plans.
monkey-wrench
Pronunciation: (mung'kē-rench"), [key]—v.t. to ruin (plans, a schedule, etc.) unavoidably or, sometimes, deliberately: The storm monkey-wrenched our plans for a picnic.
—v.i. to prevent, delay, or sabotage industrialization or development of wilderness areas, esp. through vandalism.