A character is doing something they would hate to be caught doing, whether it be illegal, out of character, or just plain strange. Naturally, that character would prefer to keep their actions hidden. So when an Innocent Bystander notices them, say, sneaking away from robbing a bank wearing nothing but a shower cap and an alligator purse, what's there to say to keep said bystander from blabbing the secret?
"Go ahead and tell everyone you know. No one will believe it."
This character has just created a Cassandra Bystander, taking Refuge in Audacity to create a Cassandra Truth, thereby ensuring their secret is kept safe and the bystander doesn't go blabbing. The circumstances around the creation of a Cassandra Bystander can be as lighthearted as a celebrity messing with a fan by doing odd things to as serious as some forms of Gaslighting. This may invoke You Wouldn't Believe Me If I Told You. If they go on to tell others, and are believed, it becomes a Tempting Fate.
Can be Played for Drama if, instead of a stranger, the bystander is someone who the character knows, or the secret that required the creation of a Cassandra Bystander is a crime committed to the bystander. Subtrope of Cassandra Truth. Compare and contrast to Cassandra Gambit, as a Cassandra Bystander is at most one or two people; more than that would mean the secret is out anyways. Compare Masquerade, which is about a conspiracy to hide something unusual from the public, and Shameful Source of Knowledge. Contrast Hidden in Plain Sight and Leave No Witnesses.
- In the Feralnette AU (Big Fat Break), Chloe witnesses Marinette tearing an akumatizing butterfly apart with her bare hands. She coldly tells the shell-shocked Spoiled Brat that "Nobody will believe you".
- Strange Times Are Upon Us: Ila'kshath, a very large Gorn, surprises a deer hunter in 1859 Pennsylvania (Time Travel was involved). In response to his CO Brokosh pointing out the possible contamination of the timeline, Ila'kshath points out that even if the boy told anyone, they'd think he was telling tall tales.
- In this Tracer x Widowmaker Overwatch fan comic strip, Sombra and Widowmaker discuss Tracer's loudness the night before, and Widowmaker says it was because she was eating Tracer's ass. She then goes on to say, bluntly, that she loves eating ass, thinks about eating ass morning, noon and night, and proudly proclaims herself "the best ass-eater". When a shaken Sombra asks why Widowmaker would tell her all this, Widowmaker smugly says, "Because no one would ever believe you."
- In Invasion of the Body Snatchers, Miles gets away from the pod people and begins trying to warn the populace, banging on cars in a maniacal way. One of the pod people says to let him go; it's not like anyone will believe him.
- An early Animorphs book has the team barely escape being boiled alive as lobsters by demorphing. The poor woman, who just had to face three crustaceans suddenly growing and gaining human body parts, is reassured by Marco telling her it was all just a nightmare, and of course she shouldn't tell anyone about it, because then she'd get locked up. Note that this is still beyond stupid on his part, because if the woman happens to be infested, the Yeerks will instantly know the "Andalites" are actually humans.
- In Brooklyn Nine-Nine, Holt claims that he injured his wrist after tripping on an uneven sidewalk. He then privately reveals to Jake that he really got hurt at a hula-hooping class, and shows Jake some photos.
Jake: Why are you telling me this?
Holt:: (deleting photos) Because no one... will ever believe you.
- In an episode of The Greatest American Hero, Bill addresses a pair of crooks who experienced Ralph's superpowers when he captured them, and tells them the last guy who blabbed about seeing a costumed man fly ended up in a straitjacket.
- In one episode of Lois & Clark, the villain learns Superman's identity through a time traveler's journal. At the end, Clark incinerates the journal and hands the villain over to the authorities, confident he won't be believed without evidence.
- In the The Twilight Zone (1959) episode "Long Live Walter Jameson," Walter Jameson is the fiancé of Sam Kittridge's daughter Susanna. After Kittridge learns that Jameson is more than two thousand years old, Sam tells Jameson that he will have to tell Susanna not to marry him. Jameson tells Sam to go ahead, because Susanna will never believe him.
- Implied to be the mastermind's plan regarding Kokichi in Danganronpa V3. After Kokichi derails everything by pretending he is the mastermind and kidnapping Kaito, the real mastermind creates a flashback light convincing everyone else that he is a Remnant of Despair and they must stand against him as symbols of hope. The point of it being, since he's a liar, no one will believe Kokichi if he denies it and the mastermind's plans can proceed uninterrupted.
- In one The Simpsons Halloween special, Homer overhears Kang and Kodos plotting an invasion. They invoke this trope, but also spray him with rum to help seal the deal.