To throw a spanner in the works

Example: “The sudden train strike threw a spanner in the works of our travel plans.”

Origin: A spanner is a wrench, and "the works" is the machinery of an industrial plant — so the image is of a heavy metal tool dropped into moving gears, where it does not merely obstruct but destroys, jamming and wrecking a mechanism that depended on nothing foreign ever entering it. The phrase is British and early twentieth-century, from the era when large factory machinery was both new enough to impress and fragile enough that a single dropped tool really could stop a production line. American English independently produced the same idea as throwing a monkey wrench in the works. The industrial-sabotage association it carries is genuine, though the parallel story about the word sabotage coming from workers throwing wooden shoes into machines is itself doubtful and probably invented after the fact.