The Cuckoo's Egg: Hunting a Hacker From a 75-Cent Error

In August 1986, Clifford Stoll was an astronomer who had just become a computer manager at Lawrence Berkeley Laboratory. On his second day, he got a small puzzle: the lab's billing books were off by 75 cents. The lab charged each research group for every second of computer time, so the books were supposed to balance. Stoll traced the gap to about nine seconds of use by an account called Hunter. Nobody had created Hunter. It had no billing address and no department behind it. Then a second accounting program showed someone logged in who was not on the list of active users. Seventy-five cents was nothing. But a stranger was inside the lab's machines, and the hunt for him would become one of the first big cases of international computer espionage.