Creating a poisonous liquid called “orange goo” many, many years (2nd edition iirc, perhaps 3.0)
Orange goo properties: At 0 degree Celsius or below orange goo is a liquid. Any warmer than that, then orange goo turns to orange gas. Ingested or inhaled, orange goo is an “instant death” poison (no save).
Anyway, I had a small rivulet of this orange goo flowing through the rooms of a dungeon. If the party followed the rivulet, then they would eventually reach the treasure room (after facing off with the BBEG).
As the party entered the dungeon, I describe the first room: It’s freezing, ice crystals have formed on the walls, patches of frozen water dot the floor, a rivulet of orange liquid flows through the middle of the room and leads to the exit. Of course, the party gathers around to inspect the rivulet
Fighter: I take my torch and put it in the liquid.
Me (just realizing the huge mistake I have made in designing the poison): The torch hits the orange goo and forms a small cloud.
…and that’s how I got my one and only TPK as a DM