I'm running an adventure for a group of kids that ends with a very unsatisfying confrontation with a dragon (the party either dies or is enslaved). So I'm replacing it with the something based on the logic problem, below. The actual logic problem won't hold up, because I can't control the exact number of characters (what if someone's unconscious or dead? What if they've rescued the prisoners? Or a player doesn't show up?). And one clever use of a spell could throw the whole thing off. But I still like the basic dilemma.
•The dragon will allow the players to load their packs with as much gold as they can carry (encumbrance rules in full effect!).
•There are three exits from the dragon's lair, which will take about 5 minutes to traverse, unecumbered.
•Two of the paths lead to doom, one to freedom.
•As many as half the party have been ensorcelled to see doom when the spot the safe exit, and safety when then spot doom.
•In fifteen minutes, the dragon will call down his lizardman army.
•Of course the dragon is lying, and will call down his army the moment he sees the party choose the right exit.
Because these are kids, and they have shown some difficulty in choosing cooperation over immediate personal gain, I'll make a pretty small number of characters ensorcelled. As long as they bother to plan, they should be able to find their way to safety, and the only real question is how much loot do they have to abandon in order to escape. But with a group of adults, the unreliability of information could be much more compelling.