I wish I'd have thought about this for the Design a Dungeon Room contest back in January!
As the party approaches the top of the stairs leading down to the next section of the dungeon, the first thing they notice are the two small shelves, one on either side of the corridor. Each is about six feet from the floor, 7 inches square and about an inch thick, made of the same stone as the walls. There is a small piece of chalk on each shelf, and remarkably little dust. Below each shelf, marked in chalk is an arrow and hash marks. On one side the arrow is pointing down the stairs, and has 23 hash marks. On the opposite side the arrow points up from the stairs, and has only 9 hash marks.
What the players can't see are the pair of invisible imps that have been tasked with keeping track of the number of groups that go into and out of the next level of the dungeon. Should players mark that they're entering the lower level, or do nothing to disturb the markings that have already been made the imps will do nothing. Should the adventurers do much of anything else the imps will cause the floor within 5 feet of the top step to become coated in oil of slipperiness/Grease spell, and then will push the adventurer who disturbed their count before flying off until they leave.