Obi-Wan: We're messing up the GM's plans again. He probably had this whole city mapped out.
GM: No, you guys have been great for my improvisation. I've learnt not to plan ahead more than ten minutes.
GM: The Prophetess of Kell reads your omens and says that each of your number is destined to find one of the Signets of Power and be a Champion of Light in the coming Holy Wars. Once all of you have the Signets the Champions of Darkness will be released, and the fate of the world shall be decided.
Player [Out of Character]: Champion of Light huh? So that explains all the stupid stuff going on lately. I ask the Prophetess where my Signet is to be found....
GM [as Prophetess]: Ritualist Timon, your Signet may be found in the ruins of the Tower of Theurgy, guarded by wards only your hand may breach.
Player [In Character]: Tower of Theurgy huh? I pull out my map and a compass. I draw a 10-mile radius circle on the map, centered on the Tower of Theurgy.
GM [looking confused]: Okay....
Player: I then ritually Geas myself to never set foot within the area represented on the map, lest I instantly die.
Player [addressing the Prophetess]: Looks like the war has been called off...
GM: *sobs quietly*
I just blew up my first gate. I'm not sure why I did it, really. I am not playing by the rules anymore.
Rose Lalonde, Homestuck
Heck, as far as I'm concerned, the presence of players is a necessary evil at best. I think most gamemasters will agree that their world functions significantly more smoothly before the PCs ever show up.
Redcloak, preface to The Order of the Stick: On the Origins of PCs
The Mouth of Sauron: Is there anyone amongst this rabble with the authority to-
"Aragorn:" Yeah, let's speed this up. (decapitates the MoS)
DM: What? You attack him? During parlay? What is wrong with you guys? This is the third time you've killed someone during negotiations!
"Legolas:" And yet they keep falling for it! It's hilarious.
DM: You're supposed to be a king! Can't you at least pretend to be one for a few seconds?
"Aragorn:" If I hadn't shot him Legolas would have.
"Legolas:" He's right, too. I was just about to announce my attack.
"Aragorn:" What's the big deal? So he's dead.
DM: I have like, four pages of dialog written for him.
"Aragorn:" Oh now I'm really glad I killed him.
Anything you expect your players to fight, they will run from or talk to. Anything you expect them to run from or talk to, they will fight.