A hallway with a floor consisting of millions of pins and needles. Anyone with even nominally protective footwear can safely tread on the array of pinpoints without injury (bare feet take 1d4 damage each step).
However, a successful Perception check DC 10 notices that each time pins in the floor are depressed, long pins extend from the walls at random places. Each step taken has a 25% chance of triggering a needle into a player in the hallway. Furthermore, if pricked by a needle from the wall, the player takes 1d6 damage (Reflex DC 15), falls prone (1d12 damage upon hitting the floor of pins) and is rendered paralyzed for 10 minutes (Fortitude DC 20 negates).
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