Picture, if you will, a painstakingly crafted horror setting. The PCs Indeah here in blue. I'll be your happy little asterisk today. PC is Player Characters for those of you not familiar with the terminology yet have narrowly escaped being burnt to a crisp and falling to their deaths. They are trapped in a hellish hole, surrounded by lifeless piles of destroyed constructs, heaped up to the point that they are a part of the terrain itself. A slight breeze carries nothing but a slight chill throughout this pit of twisted materials and bodies. There is nothing but stillness and silence, like a calm before a necromantic storm. All who enter here should be terrified.
Indeed, even the creatures traveling with the PCs are unnerved. A large wolf lets out a nervous fart to punctuate this fact. Now, the adventurers are scared and distracted. Suddenly...
...someone lets a "that's what she said" joke rip. And another. And another. Suddenly, there goes the spooky atmosphere and the session devolves into total chaos and childish giggling. Every DM's Dungeon Master, AKA Game Master (GM) or Storyteller worst nightmare. Games have distractions. It's nigh unavoidable, and it's just something that happens when you get several people together. Player 1 remembers something funny that happened to them last week and wants to tell Player 2. Player 3 makes an off-color joke. Player 4 actually passes gas. Someone wants dinner or a snack. A session will rarely go without a single interruption until the DM says "And I think that's where we'll end the session tonight."
Or, you know, an average night for our DM. He spends hours crafting these scenes and encounters for us, only to have them utterly destroyed by a stray joke gone awry or some unforeseen half-baked solution that completely derails the encounter, and, sometimes, the entire campaign. Luckily, the DM is very patient with us, and generally recovers, because, after all, even the best-laid plans can fail. Unfortunately for him, though, we are endlessly amused by conspiring against him. Not that we actually try to. The poor dear plans such intricate dungeons and complex fights, and then a roll goes astray, or ours shoot high, and suddenly the dragon we shouldn't be able to survive without casualties is face first in the ground begging for his life, while none of us are scratched (I was proud of that one...).