Everyone knows what Murphy’s Law is; everything that can go wrong will go wrong. This law is often used by novice DMs who see their PCs doing something unexpected. For example, take this actual dialog between my PCs and me during one of the first games I ever DMed (keep in mind that I was really inexperienced at the time):
PC: Since the dungeon is made out of stone, we can skip a whole bunch of rooms by stoneshaping the wall! :lightbulb
DM: Umm… you can’t! It’s completely spell resistant!
PC: Then we could always try to blow up using these explosives…
DM: It’s resistant to that too.
PC: What can we use to destroy this wall?
DM: Look, the walls are invulnerable and you’re just going to have to fight the monsters.:headexplo
If I had been more experienced, I would have let them melt the stone, but at the time I was too afraid to let them skip a single encounter. This type of method of game play is more commonly known as railroading.
The other type of DM follows the rules of agreement. The rules of agreement were created by improve actors in order to keep a scene moving. In improv, an actor will never say “no,” he will always say “yes, and”. Instead of rejecting the players more creative ideas, the DM will use their ideas and grow on them. For example, take this scene from a later campaign, when I was a bit more experienced:
DM: I know that technically you’re not allowed to stand on your own Tenser’s floating disc, but the idea that you came up with of surfing down the tunnel by using Tenser's floating disc as a hoverboard on an extremely inclined service is such a creative idea that I’ll let you take a free action point on your next attack if you meet anyone on your way down (I mean with all the momentum you have gained through the hoverboard, a bonus to attack only makes sense).
Here’s another example of the rules of agreement at work:
PC: Man, this NPC is so annoying! He probably has no life outside of helping us. He’s almost as bad as that changeling that keeps on hitting on Krunk, the barbarian. Wait a minute! If we dress up this NPC up as Krunk, maybe the changeling will think he really is Krunk. They’ll start dating, and they will finally stop annoying us!
If I was a bit less experienced, I would not allow something like this to happen because I had not planned any of this idea in advance, but instead I allowed this to take place, and all the PCs enjoyed a fun side-quest completely consisting of getting two NPCs to date.
The rules of agreement not only make the game more enjoyable, but they reward creativity and allow the PCs to break free of the track that their adventure might get stuck in.