As PCs you think that you’re invincible. You assume that no matter what the DM puts in front of you, you can defeat it. After all, you’re the heroes. Sure some fights may be tougher than others, but if the DM’s read the DMG and set up the encounter according to the rules then every fight is beatable. But what if that’s not the case?
In a recent game my DM created a super-monster that was significantly tougher than the PCs. The intent was to give the PCs an opponent that they needed to work up to. The problem was that the party (me included) assumed that if and when we found the monster that we should fight it immediately, just like we would with any other combatant. When we finally found the creature we (predictably) ran headlong into battle.
After two grueling hours of combat we still had no clue that we were in way over our heads. In fact, we thought we were doing a pretty good job of killing the monster. And then one of my companions said to the DM, “Since we’re down to just our at-will powers, are you willing to call the fight, otherwise this could drag out for a long time?” to which the DM replied, “This monster is no where near dead. I’m not calling the fight.”