In the first Harry Potter book, Hermione uses a spell to open a door. This same spell must be used later when the famous trio is searching for the Sorcerer's Stone. This is an example of Chekhov's gun. If you add one to your book, you'd better darn well be ready to fire that gun. At least, that's what Anton says.
Bang!
No, I'm not just talking gibberish. I'm talking about a quote from Anton Chekhov, considered by many literary experts to be one of the greatest short story writers in the world. Here's what he says:
"Remove everything that has no relevance to the story. If you say in the
first chapter that there is a rifle hanging on the wall, in the second
or third chapter it absolutely must go off. If it's not going to be
fired, it shouldn't be hanging there."