When a sim is turned into a toad, a dragon (and later a cat if Unleased is installed) will eat them. It was a rumour that some sims would spontaneously combust, but unfortunately no one could ever prove it.
The Sims 1 (rumoured, but never actually proven) When a sim reaches the end of their lifespan. When a sim's motives are low and they are scared by a ghost or peeking into a coffin (TS2: Nightlife). In TS1, when a sim plays with a guinea pig that has a dirty cage, or in TS2, when a sim uses the Biotech station with low motives, they will contract a disease that can be fatal if not cured. Have a sim with low motives and low Mechanical/Handiness skill try to fix something.
Put a sim in a room with a couple of rugs right by the fireplace. Put a sim in the pool and delete the ladders.
When a sim's Hunger bar goes completely red.
