TPK with a trolley...
In a modern day CoC game I ran, the party was exploring the turn of the century subway tunnels beneath New York city. They were looking for the hideout of a cult lead by a Shoggoth (though they didn't know this, and still don't

). The cult was able to repair some of the old subway trolleys in the tunnels and got them working again.
The PC investigators came across a handful of cultists (whom were dispatched) and decided to see where the trolley led. Carefully, they slowly start up the trolley and head down the dark tunnels. After a bit, they see an abandoned and partially collasped subway station. Deciding to be cautious, they stop to check it out. One of the players stays at the trolley as the other two go into the ruined area. In the search they broke open an active steam-pipe and mist begins to fill the area. The investigator at the trolley sees a shape in the mist and begins to panick and calls the group back. The investigator swears he sees more and more things in the mist and puts the trolley in full reverse. The subway trolley is now racing down the tunnel,but the mist seems to keep up with it. The group is now freaking out a bit, when suddenly all electical items in the mist cease to function. All the flashlights go out and the trolley comes to a slow stop. That's when the fun begins.
Something in the mist begins to pound on the trolley, finally breaking down the forward door. It's some sort of hound-like creature with no skin, covered in sickening goo (Hound of Tindalous), but only one of the investigators sees it then, since the rest are running out the back door of the trolley. The poor guy gets partially mauled as he runs away. So all the inverstigators are now in the subway itself, at the rear of the trolley. The mist extends around the trolley and into their area. The hound is now attacking the investigators from all angles, whittling them down. Most of the party is now extremely wounded, but as they catch sight of the hound here and there, they open fire, driving it away.
The mist recedes finally. The investigators are catching their breath in the tunnel, thankful it's over. The lights turn back on and...
The trolley starts up, set to full speed reverse, running over all the inverstigators

.