I ran a game where a doppleganger murdered the governor of the town, and pinned it on a rival assassin. He had been hired by a corrupt councilman to do the deed, and had delivered the heart of the governor to the councilman to prevent raising the dead (house rule: brain and heart had to be present for resurrection -- 2nd Ed. rules otherwise). The party (friends of the deceased) rushed to investigate his death, and through a variety of magical and psionic inquisitions, managed to discover that the assassin was a female (the assassin had taken the form of his rival, a female).
THe party also (psionically) discovered which councilman had hired the killer, so they put word out that the councilman was going to turn the assassin in. They then locked down the councilman and posted a 24-hour watch on him. The female assassin (the framed one) broke in to question the councilman and find out who was framing her. She was a "kill-only-the-evildoers" kind of assassin, and was actually neutral in alignment. After she fought the party and tried to escape, the party caught her and found out that she was the paladin's girlfriend! (That was a great reveal!)
After the interrogation, they decided that the real assassin was still out there, and they sat tight. During the inevitable poker game (why do guards, even PC's, always wind up playing cards?), one of the NPCs went to the privy, then went to check on the prisoner. Unfortunately, while in the privy, he was murdered by the doppleganger (which had plastered its malleable body against the wall of the privy -- I used a more enhanced demonic doppleganger that could conceal itself like a cloaker). He went up to "check on the prisoner" and killed him.
The weird thing was, I was playing the NPC as normal, making bad jokes, and verbally worrying about when this assassin would show his hand. He was the nookie of my wife's character, and she gave him a quick kiss just before he went up to the prisoner's room. From the way that I kissed her, she figured out that he wasn't himself, and she got the party on their feet and chasing after him. They were too late, of course, but she figured it out without a word from me. That was pretty cool.