This trap consists of a long bookshelf along one wall in a room that the PCs expect to have a secret passage. One book is particularly incongruous (Courtship rituals of the lesser Mozambican gerbil) and sticking out from the shelf slightly (Perception 13 to notice). If pulled, it causes the entire ten-foot-tall bookshelf to slam forwards onto the ground within 10′ of the wall on which the shelves are mounted. (Perception 23 to spot the hinges and possibly some very faded bloodstains on the carpet; DC 23 skill check to disable the trap or set it off slowly; in PFRPG, 6d6 bludgeoning damage with Reflex 23 to take no damage, or 20 for half damage; in M&M3E, a bludgeoning attack with a +13 bonus.)
The trap also covers the real secret exit – a trapdoor which is now under the bookshelves. (DC 28 to notice it before this, as it’s under a rug.) If through some incredible fluke a PC is standing on the trapdoor when the trap hits them, the trapdoor takes as half much damage as the PC, and if that exceeds its 15 hp, it collapses under them. It’s up to you how far they fall and what they find.