Something that I've been seeing a lot lately is that the party should be faced with encounters that they cannot win. That they should run. My question to you is how do you convey this information to the party without being ham-fisted about it? Now there are the obvious clues like the dragon being really really big or the player failing to penetrate the monster's armor after rolling a 19. But how would you subtly let your players know they're in over their heads. Have you ever found yourself in a situation where you just can't save the players from themselves?