The party is following a river when they discover a waterfall which if falling up. The party needs to get up the cliff on which the water is falling. If they choose to ride the waterfall up, they discover the hard way that it is an illusion and really is falling down. They fall, taking 10d6 damage. If they scale the cliff, they will see a man (illusionary) ride the waterfall up, making them wonder why in the world they didn't get the free ride. :)