Anyway, I came up with one I can't wait to test on my PCs.

Prelude: Ask for intelligence saves while traveling on road against a ^supposed mirage, it being a hot day after all. You can probably change it to another relevant save depending on the setting.

• Describe to your party how weary they are. I like the "use 3 different senses" rule. Except for the one who rolled the highest save. He'll be quite unaffected and energetic compared to the rest

Give at least some time between this and the actual encounter to throw off just enough those with a propensity to metagame. When everyone passes some hilltop, forest clearing, etc to give a good view of the path ahead, pass notes the party and tell them the scene going on.

• Be vague enough, only saying that you see two groups, both are significantly sized and for some reason stopped before crossing paths, with a no man's land between. Things could get bloody real soon.

Here is where the notes you passed are important. Because different PCs will see this situation differently now. Those who didn't roll the highest will see something along these lines

• The first set of notes will differentiate the caravans. The first one looks to be of merchants and traders, with the usual hired guards accompanying them. The second looks to be a convoy of outlaws. Make them menacing, and also armed. Both sides are unwilling to make the first move and attack.

Seems easy enough, defend the innocents from the big group of meanies, right?

Wrong. The note for the highest save PC will say what really is going on

• Describe the sight of two trade caravans at each others' throats. The PC is confused at this unusual sight. Maybe even write in a plausible explanation, possibly someone wanted with a bounty by one in the other caravan, with armed guards out until the tension is resolved.

Somehow, the party included, each side sees the caravan as a group of menacing goblins, bandits, etc. The reality is there is no such threat; it's an illusion. Even if the players dispel the illusions on themselves, good luck convincing either caravan to lower their arms.

TL;DR - Party sees two trade caravans. They are at each others' throats. The party/the nearest caravan sees the farther one as a group of baddies. One PC sees the real thing; that it's really an illusion and there are no bandits. Teach your murderhobos a lesson in not thinking so brashly with their swords/fireballs

Alternative TL;DR - There are weapons that are simply thoughts, attitudes, prejudices — to be found only in the minds of men. For the record, prejudices can kill — and suspicion can destroy — and a thoughtless frightened search for a scapegoat has a fallout all of its own — for the children — and the children yet unborn. And the pity of it is — that these things cannot be confined — to the Twilight Zone Forgotten Realms