All people fall over and rot.
Universal human infertility. The carnage would be easily integrated into the food chain by scavengers and maggots, but our clever machinery could cause gigantic problems for Earth. Major appliances like stoves and cars could be absorbed, but nuclear reactors would begin to melt down or explode and, tragically, no humans would be around to appreciate the fireworks. Without someone there to hear them, they might not make a sound. For miles around every reactor, a zone of death would exist for tens of thousands of years.