Minecraft wiki entry for Slime_Block, associated text: Placing carpets, rails, trapdoors, redstone repeaters or redstone comparators on a slime block does not stop mobs from bouncing and not taking fall damage. Likewise, placing a pressure plate on a slime block does not stop mobs from bouncing, but the pressure plate still activates. Half-blocks such as Cakes and slabs stop the bouncing effect.