Answer: A Flash Drive

In 2008, a U.S. military official stationed at a Department of Defense base was walking in from the parking lot and noticed a flash drive sitting on the ground. He picked the flash drive up, took it inside, and later in the day plugged it into a computer to see what was on it (ostensibly, to see if it was one of their flash drives or belonged to a co-worker).

That flash drive was actually loaded with malicious software and planted by a foreign operative. It loaded a worm, agent.btz, directly onto a computer connected to the Department of Defense network and linked to the United States Central Command. The worm exploded across the network and the Pentagon spent over a year tracking it down and purging it.

The devastating result of this simple social engineering trick led to new policies regarding removable media and, more importantly, the development of the United States Cyber Command, a division of the United States Strategic Command tasked with cyber security.