Another example: I had a group just exit a cave in the hillside that a wizard teleported them into. After a brief fight inside the cave, they exited it and heard some rocks being moved above them. Over a small crest they see a Hill Giant, minding its own business, playing with some rocks. Then they see the Hill Giant playing with a goblin, and see him toss the goblin into its bag of rocks. This group of characters, 5 in all, averaged 3rd level. The Hill Giant had a CR of 8. As players go, they flat out attacked the creature because it was there. By the end of the battle, there were two survivors, one at -HP, the other three were dead, the Hill Giant was dead, and the two survivors had no way of carrying the 3 dead bodies with them to the nearest town, which they didn't know where it was anyways. Then this group got pissed at me because they felt I was unfair. I told them that their characters live in a WORLD, and in this WORLD things beyond their capacity to handle do exist. Plus, I told them I was testing their intelligence and see if they would actually run from something than attack it. They didn't like that too much. So, after attacking a Hill Giant that had absolutely no knowledge of their existence, the campaign ended right there. Was that my fault that they acted rashly and didn't think about it? I don't think it was.