Something fantastic enough that even non-experts would say is impossible in the real world, but in the fictional world, it's plausible enough that the audience and characters buy it.
Until there comes a point where either the hero or the villain says "Did you really believe that existed?"
- In the Doctor Who episode "Amy's Choice", The Doctor realizes that the reality of the freezing Tardis presented by the Dream Lord is also a dream partly because "who ever heard of a cold star?"
- In the Sherlock Episode "The Reichenbach Fall", Jim Moriarty claims to have a few lines of code that can act as a key to any secure computer system in the world, but it's just a ruse because what idiot would believe you could hack any system with a universal snippet of code?
- An entire season of Frisky Dingo revolved around Killface and Xander campaigning for the US Presidency. In the finale, it was revealed that of course neither of them could be president, as Killface was not born in the US and Xander was under 35.
- In Ultimate Fantastic Four, Zombie!Reed Richards appears to have built a teleporter out of bamboo and coconuts, explaining at length about making a biological computer out of his wife's hair before the Four vanish, but it's soon revealed that Zombie!Sue just made them invisible so they could ambush the people after them, and Reed mocks them for believing that could work.
- Star Trek Log Seven by Alan Dean Foster, a novelization of the Star Trek: The Animated Series episode "The Counter-Clock Incident". In the original episode the Enterprise enters an Alternate Universe where time runs backwards, including living beings that are "born" as corpses, grow younger during their lives and "die" after they return to the womb. Later on, in the additional material Mr. Foster created, the Enterprise crew learns that their entire experience in the other universe was created by a group of highly advanced beings. Those beings can't believe that the Enterprise crew fell for the trick, because the premise was so unbelievable.
- In Whatever Happened To The Man of Tomorrow, Alan Moore's last Superman story, he had Mxyzptlk show up and say basically "did you really think a fifth dimensional being would look like a funny man in a derby hat" and reveals Mxyzptlk to be a Cosmic Horror.
If it's the hero that calls out the bluff, he's Spotting the Thread
, but the specific focus of this trope is that there's an object or event that seems plausible for the world, but gets called out as actually being just as impossible as you'd think. Compare You Watch Too Much X
and Scooby-Doo Hoax