"Consider the nation's dams, on average a half-century old. Despite their monumental size, the dams can be weakened by foraging gophers and squirrels, whose holes undermine the foundations. Or even by simple operator error. A major gate at Folsom Dam in California burst in 1995 after the wrong lubricant was used on its gears.
Tree stumps and rusting pipes can undermine levees in Sacramento. Water systems in Alaska and Washington State depend on wood pipes dating back to pioneer days. And locks on inland shipping routes can be weakened by simple flotsam like discarded tires."
From flooding in levees to broken links in a subway's power system, seemingly minor faults can have devastating and expensive results.