I was DMing a personal "update to 3.5" of the OD&D module B3:"Palace of the Silver Princess"
For who don't know, everyone inside the castle is petrified, and a strange, red glowing magic corrupted everything, and attracted monsters.
They worked well their way up trough the first level, and climbed up, finding the Doppleganger that is impersonating the High Mage of the Palace. It bluffed away well, expecially because no-one dubted him. He decided to go with'em "to help". Noone fizzled when, during a fight, he said some mumbo-jumbo, made his hand dark and clawed and fighted in front line. He said it was an ancient spell and that he doesn't want to use more "explosive" spell not to damage the place.
They enter in a room, with a fireplace in the end of the room. The Maug of the group (who FINALLY ended te HD+LA and was completed) decided to toss an eye inside. It was invested by flames. A Ibrandlin came out of the fireplace and... well, the Maug died.
It was a construct, so, end of the story.
BUT then, a lamp turned on in my mind.
The "High Mage" examinated the rests and said something about a memory stone that allow to rebuild the poor guy. He made all the group (and the 2 infiltred thief..I've said that there were 2 of them? 2 poor damisel in distress?) to transport all the rests to another room. He said that he would stay there until he finished.
In real, cigarette pause. I take the Maug player by a side and said to him the truth: he will play the doppleganger and he will try to kill everyone else at the first good chance.
When we restarted, it arrived the "Maug", only smaller (medium size), saying something about that part of the material was damaged too much, and that "the Good High Mage" had to reshape him smaller. The Mage? Oh, was sleeping to stay better.
They go on.... but are depleted of hp. They decide to go to sleep.
The player look to me, and nod. He was always the one on guard, beeing a construct and not needing to sleep.
I start from a random player, and I said "make a listen check". He fails.
Then one of the NPC. She fails.
The second, pass. And open his eyes. And he see a 2 handed sword fall on his head. Roll away. Wake everyone with a scream. Destroy the Maug...and found a player dead, and one of the thief dead.
Investigation trough the palace... found the petrified body of the archmage.
They hated me soo much. But...they also remember very well that game, and we joke again on it.
From then up to now...they are MORE diffident on the NPC.