This happened in our last Warhammer Fantasy campaign. We had been involved in a witch hunt in a small village, watching from the sidelines growing increasingly uneasy by the way the witch hunter did things given he was a lot more fanatical than our rather moderate party. Then dark elves attack and all that is forgotten for a moment as the whole village, all the witch hunter’s men and our adventurer party all rally to fight against a common enemy. We win a decisive victory but in the aftermath the witch hunter’s madness reaches new heights and he wants to basically wipe the village off the map, something we won’t allow. We reluctantly kill him and his remaining followers, but are feeling really up the shit creek without a paddle now - you just don’t kill witch hunters and live happily ever after.
Our Agent is looking at the witch hunter’s corpse and the player says: “oh, I put the chaos symbol I’ve been carrying with me into his pockets and then let our Sigmarite priest find it.” Every player goes silent and then starts laughing - he has been carrying that thing in his pockets in secret since the first adventure, roughly 20 gaming sessions. He easily succeeds in fooling my Int 2 priest, who raises the damning symbol high up in the air and starts proclaiming the dead witch hunter a traitor, a heretic and an agent of the Chaos, believing in every word he says.
Saved! He was evil guy after all.