At the height of its power, the Pinkerton detective agency was the largest in the world.
Allan Pinkerton is believed to have foiled an assassination plot against Abraham Lincoln. After this, the President hired Pinkerton agents for his personal security during the Civil War.
The agency's reputation fell during the nineteenth century, as they were hired as "union busters." They would infiltrate labor unions in order to disrupt activity and gather information. They became a corporate disruption force.
The French film Farewell is loosely based on the later life of Vladimir Vetrov- a KGB agent. He passed along approximately four thousand secret Soviet documents to the French from 1981 to 1982.
Farewell does romanticize him to an extent. The real Vladimir Vetrov stabbed his mistress during an argument in his car. The drunken agent then fatally stabbed a police officer who came to check up on them.
He was sentenced to ten years. But, while in prison, he couldn't resist bragging in his outgoing correspondence the he had "been involved in something big." His secret was pieced together by Soviet intelligence, and his ten year sentence turned into an execution.
In the year 774 AD, Telerig Khan of Bulgaria sent word of his intention to flee Bulgaria for Constantinople, the capital of the Byzantine empire. The Byzantines were on the verge of war with the Bulgarians. He requested safe passage, and a list of Byzantine people who might help him flee Bulgaria. The emperor, Constantine V, apparently trusted Telerig, and sent the list.
Telerig Khan stayed, and the Byzantine agents in Bulgaria were rounded up and executed.