The more deadly the trap, the easier it is to evade. Think of the ballista trap, a trip wire launches a massive bolt guaranteed to skew anyone it hit to death. But, it only hits someone if they're standing in its path. So duck and do the 'penitent man shall pass' and everyone will be okay.
Given thieves' and magicians' and clerics' ability to detect poisons, traps, predatory lionsloans, and what not, it is a wonder that any trap ever hurts anyone.
So the trap that isn't a trap. A cream pie in the face, or a moat that the raiders have to swim through to continue. The filling or the water might not be deadly, could be half of binary poison that is administrated by a later trap. Or it's pheromones of a Martian Thark in heat and the some of the dungeon's guardians are bull Tharks. One whiff of you, and it's death by bunga-bunga.
Similarly harmless mold spores could be seeded along the dungeon. But they react at a later part, deeper in the dungeon, poisoning the water supply or busting into flame consuming everyone's food, spare clothes, and lamp oil.
Things like this would consume precious resources that raiders need to steal your loot without really doing any direct harm to them.