In module B2 “Keep on the Borderlands” for early edition D&D, there is an encounter area that the locals call “The Caves of Chaos” where much of the adventuring is assumed to take place (there are other small opportunities in the keep itself, but it's mostly up to the DM to generate action there). The titular caves are set in the walls of a vale, and each series is inhabited by members of the typical adversarial “monster” humanoid species, one with goblins, one with orcs, etc.
So imagine you are a young GM, and the party comes to these caves, and they quite cleverly think… “What if we just smoke the monster's out of the cave and wait here at the mouth of the cave to ambush them?” They say that they intend to do this. As the DM of the game… do you let it work?
This situation happened to me in a play of that module, and I have heard it repeated at random by at least three other DMs who have run the module. And in many cases, an issue occcurs.