I definitely failed to warn the PCs they were going to the dangerous dungeon. I could have made sure they had a cohort to take along. The reason I didn't warn them is that the start of the dungeon was low-key, and the encounter that nearly killed them was potentially only half of a larger encounter, so I thought it was safe to let them go in without warning. But improved grab is, as always, a character killer.
Later on in the campaign, when the PCs can succeeed against that location, this will very likely be a legendary story. When the PCs know that the world can kill them -- and
will kill them, if they don't play well -- in-game accomplishments become far more meaningful, IME and IMHO.
As has been mentioned upthread, a part of your DM prep time should be devoted to deciding how the world reacts to the PCs. If you do not, your players won't get the immediate benefit of seeing that their actions change the world.
Some suggestions:
1. Plant hooks about what treasures are where. "Legend says that the Staff of Horus is lost in the Dungeon of Boiling Warts." Giving the players definite goals to achieve within a setting is a good thing.
2. Let the mayor, tired of nagging the PCs, hire some other would-be adventurers. Over time, she hires a sherrif. Based on what she has told the new adventurers, their opinions of the old adventurers may (initially) be rather poor.....
3. As above, but give one of the NPC adventurers a cool magic item. When said adventurer is lost in the Dungeon of Boiling Warts, the PCs may well want to try to recover the item. The question then becomes.....does the item belong to the PCs or to the adventurer's heirs? The answer will affect what happens when PCs die and their gear is recovered by NPCs.......
4. In addition to random encounters, write up a few small lairs that you can drop into the campaign world as needed.
Do not let the players know that you are placing these lairs during the game -- they were always there! One placed, mark 'em on the map so you remember they are there.
RC