The Rule
Don't set up things without paying them off.
"This disproves that ancient literary technique...Chekov's Voltron, wherein if you have guys who might form Voltron, they'll form Voltron." — Conor Lastowka
The trope namer is from books #7 The Mister and #24 Artemis: Bleriana was a trafficked Albanian girl used to give The Mister some gravitas, who vanished completely without explanation or follow-up, becoming a verb, "to bleriana".
Flatcap refers to Andy Weir, who built up a plot point involving a condom that then gets dropped without mention.
Counter-Example
That is when I saw a spot of blood on the floor. That was something I couldn't put back where it belonged. There were many more farther on. And I don't mean to draw out the suspense here to no purpose, to give readers a frisson, to let them suppose that I would find Mary Kathleen with her hands cut off, waving her bloody stumps at me. She had in fact been sideswiped by a Checker cab on Vanderbilt Avenue, and had refused medical attention, saying that she was fine, just fine.
— Kurt Vonnegut, Jailbird
Vonnegut has relayed, throughout the book, an urban legend about a maniac who chops off women's hands, and he knows exactly what the reader is going to think here, and immediately defuses it. Honestly, it still kind of pissed me off, but that may have been Vonnegut's purpose.
Commentary
One of the most gratifying elements of a reader's experience is when something mentioned and remembered from earlier on in the story makes a reappearance. In a nutshell: The author has set something up and then paid it off. This can be done poorly in many ways: If everything we see in a realistic story is completely resolved, you have a "Hollywood Ending"—the reader may feel manipulated. If the author presents something with bright lights and big takes to the audience while he says "Get it?", the reader may feel insulted.
However, little is worse than setting something up and then NOT paying it off. The reader may feel insulted that you didn't even care him enough to try to manipulate him, or worse, that you just wasted his time.