Different Lies To Find The Spy

"I told Varys I was giving the princess to the Greyjoys. I told Littlefinger that I planned to wed her to Robin Arryn. I told no one that I was offering her to the Dornish, no one but you."

- Tyrion Lannister

Someone is a spy. But how to find them? This trope is about one solution. Tell multiple people different, but similar lies, and then see how the person the spy works for reacts. It may be worth noting that doing this doesn't inherently prove that the information leak is actively trying to act as a spy, maybe they just have Loose Lips and their spouse is the actual spy or something. In fiction, however, usually the leaker is a spy.

Fan Works

 * Jenny does this in A Young Girl's Delinquency Record. After she accuses Stevie, Stevie makes a run for it. Jenny interprets the running as a confirmation that Stevie was an informant for the police (instead of there being a coincidence), and then presumably kills him in front of her other subordinates to make an example of him. At the very least, she hurts him enough to get blood on her knuckles.

Live-Action TV

 * Tyrion Lannister determined that Pycelle works for Cersei this way in Game of Thrones, as shown in the page quote.

Web Original

 * Done in Fenspace during the "Boskone War", in order to root out which group in the allies was leaking secrets to the enemy. Each group was told to rendezvous at a different location; when the enemy forces showed up early at one location to ambush the allies, the allies knew where the leak was.

Western Animation

 * Tina Belcher did this in the Bob's Burgers episode "Tina Tailor Soldier Spy ".

Real Life

 * Who leaked the spoiler that in Star Trek II: The Wrath of Khan? Thanks to somebody making sure each script had a unique code - in essence, something different was told to each person involved - we know that it was.
 * The Electronic Frontier Foundation asks, with good reason, "Is Your Printer Spying On You?"