In November 1988, a graduate student at Cornell University named Robert Morris, Jr. inadvertently sparked a national crisis by unleashing a self-replicating computer worm on a VAX 11/750 computer in the Massachusetts Institute of Technology’s Artificial Intelligence Lab. Morris had no malicious intent; it was merely a scientific experiment to see how many computers he could infect. But he made a grievous error, setting his reinfection rate much too high. The worm spread so rapidly that it brought down the entire computer network at Cornell University, crippled those at several other universities, and even infiltrated the computers at Los Alamos and Livermore National Laboratories.
Source: Is cybersecurity an unsolvable problem? | Ars Technica