Quick Player Scare

It's easy to put together a trap that is inescapable, but not lethal--a good example being a magically concealed pit, that when fallen in, has iron bars slide over the top, and the pit fills with water (or whipped cream, even), then just before certain death, the water drains, a ladder appears, and at the top of the ladder is a note reading "You have been warned. The next trap will be fatal. Go home." Such a trick is sure to plant doubts into the minds of the players, and even if the trap is completely isolated and nothing comes of the warning, it will get the players on edge and make them cautious.