The last dungeon I ran made my players somewhat concerned
I made a Homebrew system for magical radiation poisoning where the more Rads a character gets in their lifetime, the worse the effects are. I’d never intended to actually flesh out the system, but our ranger wanted to keep using the radioactive flight harness (basically ODM gear from Attack on Titan, but running on spent Magical Uranium(tm) fuel rods) even after learning it kills its users. So I had to come up with the actual mechanics.
And if I’m writing the system, I may as well use it in a fully optional side dungeon full of radioactive undead dinosaurs. (Fully optional meaning they only went in for the treasures within. No quest hook or heroic impetus, just them being loot goblins. I wouldn’t employ such a punishing mechanic on a main quest location.)