A few episodes back, in MerelyCompetent's weekly game:
Our heroes ran into the PC meatshield's bigger, badder brother: A half-dragon, half-ogre mage (they think) monk/multi-classed monstrosity. Average PC level: 11. EL: Higher than them, but not into certain TPK. To make a long story short, they went through the mooks like a flametongue through butter, saw big brother waiting for them, and ran away screaming like little girls. Big brother had terrain (with more mooks), planning, and Dad's spells backing him up. He also ripped a big old oak tree out of the ground one-handed to use as a makeshift club. As a DM, I didn't expect that to scare them quite so badly
Several more episodes back:
Our heroes stomped the living Arrr! out of some orc barbarian mercs with magical support, accepted the survivors' surrenders, and made them promise to never return to civilized lands again. They ambushed the poor orcs rather cleverly (bloody bow-specializing ranger gacked the sentries while the half-dragon meatshield snuck in and bent the magical support into pretzels, with the help of the cleric's & wizard's spells). Average PC level: 8. EL: Equivalent to PCs at the start. PCs planned and executed a skillful ambush, using terrain to their advantage. The orcs had chosen an excellent and defensible camping site with sentries and random patrols, but the PCs successfully scouted them out and used magic + skills to overcome the orcs' advantages.
I look forward to them luring their opponents into chasing them off into the trees. That ranger will make mincemeat out of them, if the monk doesn't get them first. And the cleric just discovered the joys of Divine Power + Righteous Might.
PC moral of this story: My players wisely included Retreating as an option in their bag of tricks. They convinced the NPC wizard to memorize a few Wall spells (and always keeps one on a scroll), the PC cleric keeps Sanctuary and a few others in reserve (some on scrolls), the half-dragon monk pulls bodies out of trouble, the ranger lays down covering fire, etc. Metagame-wise, the character builds reflect this. This is with the understanding that they may find themselves in a spot they can't retreat from, so negotiating a surrender is a *good* thing.
DM moral of this story: DMs can encourage or discourage certain kinds of behavior in the players. I strongly encourage my players, in this heroic high magic campaign, to get away from the fight-to-the-death mentality. If they slaughter surrendered opponents all the time, word gets around and NPCs stop being hospitable, plus enemies stop surrendering. If they accept surrenders, most of their enemies will be glad to live through it -- and will run away if encountered again. There are, however, the exceptions to keep the PCs on their toes. And it doesn't prevent them from biting off more than they can chew. The game is, after all, about the players' choices in the DM's world.