Milan Kundera, the reclusive author of The Unbearable Lightness of Being--one of my all time favorite novels--reportedly snitched on a western spy. AP writes:
...Kundera in 1950 informed on Miroslav Dvoracek, who had been recruited in Germany by the Czech emigre intelligence network to work as a spy against the Communist regime.
Dvoracek visited a woman in Prague and left a suitcase in her apartment. She told her boyfriend, who later told Kundera, and Kundera went to the police.
Dvoracek was arrested when he came to collect the suitcase. He was later sentenced to 22 years in prison and eventually served 14, working in uranium mines.