Maybe not exactly a Chekhov’s gun situation but still very similar.
Also, spoilers if you’re planning to do the Wild Sheep Chase one off adventure.
It was my first time DMing and naturally the party starts by going to the tavern. After some shenanigans and arguments with the barkeep, our ranger decides to mess with a passed out drunk. After trying (and failing) to wake this guy up, ranger decides to pickpocket him. Since it was just some bum, I decided he found a few coppers and some lint. The ranger takes both.
Anyway, after progressing through the quest and reaching the final forest lair, they meet the boss. The boss is a caster and has a transmutation staff. Ultimately during the final boss fight the caster uses the staff on a bed and turns it into a dragon wyrmling. After some other goons weaken the party, the party is not doing so hot with the bed-dragon. After one death and what looks to be a final stand, the ranger’s turn starts.
The ranger asks a special request. “Can I use the lint on an arrow, and light it on fire?” I was floored at this, I instantly knew what was about to happen. I said sure, he made his shot and was able to set the bed-dragon (which by nature is made of the same material as a bed) and it goes up in flames, taking down the beast and seriously hurting the boss caster.
They managed to come out of the fight victorious and that was that. Often it’s the players forgetting about important items, but this time that was flipped on me. Just a funny creative solution by one of my players.